The NY Times tells us that they have gotten a copy of a January memo from Trump's legal team (at the time) to Mueller arguing that Mueller can not subpoena Trump. To their credit, they note in passing that a Mueller subpoena is not a slam-dunk:
Mr. Trump’s broad interpretation of executive authority is novel and is likely to be tested if a court battle ensues over whether he could be ordered to answer questions. It is unclear how that fight, should the case reach that point, would play out. A spokesman for Mr. Mueller declined to comment.
It was relatively easy to find legal experts saying that this is a fight Trump won't win when the issue arose recently. Vox, reliably lefty, contacted nine legal experts and the general theme was pro-Resistance. Keith Whittington stood out as wondering just how clear-cut Mueller's power to subpoena would be; he provided more at Lawfare. Do let me add that all this ought to be subject to a caveat about dramatic new information from Mueller that could sway the undecided, uninterested or confused.
Also at Lawfare, Benjamin Wittes and Steve Vladek gave Mueller a strong 'definitely maybe':
The bottom line, in our view, is that Mueller would probably prevail if and when a battle over a grand-jury subpoena makes its way into court. But it is not a sure thing, and the president has plausible arguments available to him that a court would have to work through before enforcing a subpoena for his testimony.
So, better odds of prevailing than the Cavaliers but not as good as the Yankee's chance of winning the AL East (currently 57% at FanGraphs).
Stuart Taylor, writing at The Weekly Standard, has similar handicapping:
Mueller’s risk in starting a subpoena fight is that he might win less in the Supreme Court than he could have gotten in negotiations, or even lose entirely.
This last possibility is something that Trump’s critics greatly underestimate. The popular analysis is that the Supreme Court’s decisions in U.S. v. Nixon (1974) and Clinton v. Jones (1997) require the president to obey a subpoena to testify before Mueller’s grand jury. Harvard’s Laurence Tribe exemplifies those who hold such presumptions. “The Supreme Court held in the Nixon Tapes Case that executive privilege cannot overcome a grand jury subpoena,” he told Business Insider in March. “So Trump would have to answer every question or be held in contempt—unless he takes the Fifth Amendment.”