He didn't do either, and in the process surprised (or dismayed) longtime court observers of every political stripe.
Those two outcomes in the finals days of his seventh year on the court offer some clues for reassessing what kind of chief justice Roberts is and intends to be. Is he no longer the rock-ribbed conservative loved by supporters and jeered by opponents? Has he become a pragmatic leader mindful of the court's place in history? Is he more canny, but still solidly conservative?
The measure of a justice is best taken after decades of service, rather than a few years. At age 57, Roberts could lead the court for another quarter-century.
But at the very least, the end of the Roberts Court's most consequential term already is leading to revised, and in some cases more nuanced, appraisals of his leadership.
Erwin Chemerinsky, a liberal scholar who is dean of the law school at the University of California at Irvine, announced that the era of the Roberts Court had begun. "He authored the opinion in the most important case in his seven years on the court, and did so against what was expected," Chemerinsky said.
In truth, Roberts' vote to uphold President Barack Obama's health care law was not so much a surprise.
He long had been counted among the possible votes to uphold the law. But it was widely assumed that if Roberts ultimately voted for it, so too would Anthony Kennedy, most often the decisive vote in closely fought cases. It was only the second time in his tenure that Roberts provided the deciding vote for the side favored by the court's liberals.
Up until now, it had been the Kennedy Court, Chemerinsky said, "This year, it was the Roberts Court."
Had Roberts gone the other way, the court would have wiped away the entire health care overhaul, which is the outcome embraced by dissenting Justices Samuel Alito, Antonin Scalia, Clarence Thomas and Kennedy.
Instead, he said the individual insurance requirement at the heart of the law could be upheld as a tax. At the same time, he rejected administration arguments that the mandate was justified by the Constitution's clause giving Congress power over interstate commerce, which has been the authority for most federal programs since the New Deal.
Some legal scholars suggest Roberts produced an essentially conservative opinion with a liberal outcome.
Neal Katyal, the former acting solicitor general who argued the Obama administration's side in the health care cases in several appeals courts, said that Roberts' majority opinion opened the door to potentially important changes in the law that could restrict federal power as it has been understood since the New Deal.
Ilya Somin, a George Mason University law professor, said on the Volokh Conspiracy legal blog that the health care case "gives supporters of limits on federal power some useful ammunition, despite also dealing us a painful defeat."
In addition, Roberts' ruling has helped refocus the public debate over the law and gave Republican opponents ammunition for calling it a big tax increase they would try to repeal.
Just three days earlier, Roberts, joined by Kennedy this time, sided with three liberal justices in the Arizona immigration case. Justice Elena Kagan, one of the court's liberals, did not take part because of her previous involvement as solicitor general, when she was the Obama administration's top Supreme Court lawyer.
Roberts joined in every word of Kennedy's majority opinion that struck down three contentious provisions of the law, but allowed one to go forward — the requirement that police check the immigration status of people they stop for other reasons.
Roberts could have sided with the conservatives who favored allowing other provisions to take effect, but that would have left the court divided 4-4, a distasteful outcome because it amounts to a waste of the justices' time.
What's more, Roberts might have reasoned that Kagan probably would side with the other liberal justices and Kennedy in some future immigration case over similar state laws that she could take part in. Such a case down the road probably would produce an outcome similar to what the court achieved last week in the Arizona case. But if Roberts stuck with the conservative justices, that scenario would have deprived him of a chance to try to influence the majority opinion.
Yet as the court followed its usual practice of issuing a torrent of opinions just days before the justices scatter for the summer, Roberts did not exactly abandon his frequent allies on the right.
He wrote a sharp dissent from Kagan's majority opinion barring mandatory sentences of life without possibility of parole for people younger than 18. The chief justice and the other conservatives rebuffed pleas from supporters of campaign spending limits and from liberal justices to take a fresh look at the 2-year-old Citizens United decision, perhaps the most politically unpopular decision of Roberts' tenure.
The court struck down Montana's limits on corporate campaign spending because the majority, including Roberts, said the state law conflicted with the Citizens United decision.
Montana Attorney General Steve Bullock called the court just "another political body." Liberal groups bemoaned the loss of a chance to carve out state exceptions to the Citizens United ruling that unleashed a tidal wave of big money contributions during this election year.
Roberts has called for consensus in judicial decisions since he arrived at the court in September 2005. He has advocated judicial modesty, though his critics insist he himself cast that aspiration aside in Citizens United and other high-profile cases involving abortion rights, race and gun rights.
Other predictions of furious discord among the ideologically divided justices in big cases were sometimes proved wrong. The justices avoided a major confrontation over the landmark civil-rights-era Voting Rights Act in an 8-1 decision that resolved the case without reaching disputed constitutional issues.
When the court heard a dispute over congressional redistricting maps in Texas that essentially pitted Republicans against Latinos and African-Americans who argued for greater representation, the court seemed headed for its typical conservative-liberal split. Instead, 11 days after hearing arguments, the court returned a consensus, unsigned opinion that gave both sides some of what they wanted.
Paul Clement, who argued the health care, immigration and redistricting cases, said the Texas case was a remarkable accomplishment for Roberts. "It was not foreordained as a unanimous decision. One has to assume it was largely due to the leadership of the chief justice," Clement said.
So what does the future hold?
The court already has agreed to hear a challenge to the University of Texas' affirmative action program. A new case involving the Voting Rights Act and challenges to restrictions on gay marriage are close behind.
Roberts has voiced serious reservations about racial preferences in government programs. In 2007, he declared in ruling against public school system programs to promote integration that "the way to stop discrimination on the basis of race is to stop discriminating on the basis of race."
Ben Wittes, a legal scholar at the Brookings Institution think tank in Washington, cautioned against grand re-evaluations of Roberts because of the health care case.
"Roberts is, to be sure, no shrinking violet about ideologically divided opinions when, in his view, the law compels them, but he apparently has a more flexible view than do his conservative colleagues concerning the difference for constitutional purposes between a penalty and a tax," Wittes said. "In other words, don't be too surprised if Roberts next terms looks like a conservative again. He actually did not stray very far from where the other four conservatives ended up in this case — just over a consequential line."
Roberts repeated his desire to have the court adhere to judicial modesty Thursday at the start of his health care opinion.
"We do not consider whether the act embodies sound policies. That judgment is entrusted to the nation's elected leaders. We ask only whether Congress has the power under the Constitution to enact the challenged provisions," he wrote.
Thursday's dissenters accused Roberts of "vast judicial overreaching." Several years ago, Justice Stephen Breyer said of Roberts and the other conservatives at the end of an especially contentious term with an unusually high number of 5-4 decisions that "never have so few done so much" in so little time.
Roberts has been mocked for comparing a justice to a baseball umpire calling balls and strikes.
But umpires often take it as a compliment when both teams are griping about their rulings.
Supreme Court interactive: http://hosted.ap.org/interactives/2012/supreme-court