I think it boils down to a few things (some are obvious and have been discussed already).
One of those things is how he feels about being a winner. Does he want to be a winner or does he want to gravy train.
Now I'm not saying a team with Nurkic, Damian, CJ and Anthony is going to beat Golden State, but I think they'd have a better chance than Houston would with Paul, Harden and Anthony.
Damian and CJ is about par with Harden and Paul, offensively. Defensively, Paul is better.
The issue would be how he fits in with both. Would he be able to accept having Paul and Harden do most of the offense? Or CJ and Damian? Would the idea of being around 3 other starters of note (CJ, Damian and Nurk) be more appealing than Paul and Harden?
Honestly, I think the Blazers squad (CJ, Dame, Nurkic and Anthony) are much better suited for a title run than the Houston squad would be (Harden, Paul and Anthony). Besides the age factor, I think both CJ and Damian have shown a great willingness to let others be the guys in the spotlight.
But I think that to Anthony, the idea of playing with the popular "superstars" is too much to pass up. Especially if/when they don't win, he won't be pegged as the reason.