I think that if you can bother with custom scenery and stuff like that, then you should go for RCT3. Just remeber though, that Tomes is extremely good at this, and it will take a LOT of time to master it and get that good. It's also VERY time consuming, so if you just want to get into a game and just play for the fun of it, you should choose the other one. It's much more rewarding for a beginner and looks good without all the custom scenery and (atleast to me) more fun to play.
Basically:
RCT3 + custom scenery + a lot of time = Realism and beauty
RCT2 (or RCT1 for that matter) + less time + ideas = fun
All of this is a personal opinion ofcourse, others might say something else. Hope this helps, anyway, have fun playing whatever game you choose.