The differences between the first game and the second are purely cosmetic.
In fact, I consider RCT 2 to be more of an expansion pack to the first game.
However, RCT 2 has more ride types, more customization, smarter guests, a couple of new modes, etc.
However, if your interested in the series, the third installement should prove to be just the ticket. It should be coming out just in time for the Holidays. It will have a brand new 3d engine, as well as a night time mode.
Can't wait!