The good parts of STW2:
-AI is better. Better on battlefield, and smarter on campaign. Defends cities better, acts more aggressively. Legendary difficulty is really mean!
-Better diplomacy. AI is a lot more reasonable.
-Less agents. Playing MTW2 again recently I was really annoyed by how many agents I need to move. By STW2 they cut out princesses, merchants and diplomats as agents leaving just spies, priests and assassins.
-I also like the new building style where cities have slots so specialization is more straightforward.
So there have been some significant improvements in the engine since STW2 so it will be great to go back to Rome.
I hope Roman politics will be a bit deeper if you choose to play it, it was kind of weak in RTW where you just chose 1 of 3 factions that you ran like a kingdom and then the senate acted like a pope. The senate should be A LOT more significant. If you don't want to deal with that then just go play some non-Roman nation, but Republican Rome should be special and different.