The weird thing is, both series I used to be REALLY into. But after a few sequels, I kind of lost interest in both of them. Sim City got bigger and more graphically intense, but for some reason the loss of the more accessible quality, of the older sim city games, kind of made it more of a chore. Theme Park I used to adore, back with the first one, but the later ones got more graphically intense but didn't overall add much to the package. Not to mention, the original Theme Park (with exception of having the most insanely addictive (more addictive than "It's a Small World") SONG for the bouncy (jump) attraction, hasn't exactly lasted the test of time(It's rife with inconsistant bugs that newer games like Rollercoaster Tycoon just doesn't have). I haven't played the game for years, but I remember the whole "bouncy ride" song more than the average person knows the "Feel the Fizz" song.
However, I think RollerCoaster tycoon has completely taken over my Sim City/Theme Park addiction. It's the one game that can still be played with such an easy to jump into fashion. It doesn't require as much management (unless you want to), nor does it overdo the fancy aspects to the point it takes away from interface simplicity and speed. The user-made variety you can spin on your rides (and themes) in just unmatched. Not only can you make it beautiful, you are given the highest degree of park customization, in the genre. Plus, it contains IMO, the best (easy on the user) multi-windowed interface. The convenience levels of handling aspects of your park is nearly unrivaled. Now if the new Roller Coaster Tycoon (3D) doesn't make the mistakes that Theme Park 3D did, I'll be all over it. I just worry that something will be lost in the translation as I'm not sure the degree of user freedom and convenience is ready to be done in 3D as it was in 2D.