I guess it depends if you like theme parks or zoos better.
Zoo Tycoon doesn't have rides. There are zoo exhibits that you arrange and care for the animals inside. You can set up orca or dolphin shows with the marine expansion pack. Other than that, it's about keeping your zoo afloat.
Roller Coaster Tycoon is a theme park simulator. You build a theme park. You can build your own roller coasters according to physics, or you can choose the pre-built ones. Other rides are primarily pre-created (except a train that goes around the park, if you want one, etc.). There is an expansion pack that includes some animals, but it's not really a zoo simulator.
I like Zoo Tycoon better personally, but I like animals better than roller coasters, lol.