Too hard to answer right now. The talking heads keep bringing up that NE is getting a former MVP that accounted for 45 TD on a 15-1 team that went to the Super Bowl. While that is true, they are also getting a guy that couldn't finish the last two seasons, went 0-8 over that stretch, and has taken a pounding over the years.
So until we have any idea what shape Cam is in health wise, if the pounding he has taken hasn't sapped some of his game, and if he can pick up the complex NE offense (even if they modify it some), we have no idea what Cam Newton the Patriots are getting. Literally anything from Cam not making the Week 1 roster to him starting every game all season is on the table. I doubt either on of those is the actual outcome.
If I had to guess, at this point Stidham seems like he is in a better position to play Week 1 (not based on being a better player but based on having a better grasp of the offense). That doesn't mean as the season goes on that Cam doesn't take the job, but if I had to fathom a guess (not knowing anything more about Cam), I would put forth that Stidham would be a better option to start against MIA. The other potential question is do they proceed with the same offense they ran with Brady (probably not) . . . and do they try to implement a new scheme this year that fits BOTH Newton and Stidham (I am not sure there is an offensive model that fits both of them).
Again, everything is subject to change with more information to add into the mix . . .