Don't forget Church Street Station (including Rosie O'Grady's Goodtime Emporium) is no longer open, which HAS had an effect on the downtown scene. There are still some great clubs, bars and restaurants, but they are more spread out now. If you like clubbing, there are several very lively options, while the bar scene is great (if a little raucous on Friday and Saturday). Orange Street is basically the centre of the city nightlife area.
For a really special oneoff night out, however, I might be more inclined to take in Cirque du Soleil at Downtown Disney, dinner at somewhere like Wolfgang Puck's or House of Blues, and then a few hours at Pleasure Island.
CityWalk is a genuine alternative, with a slightly more eclectic selection of bars and clubs (and several outstanding restaurants - Emeril's and Latin Quarter).
Then the POinte*Orlando has a fair bit to offer, too, with several more excellent restaurants, two discos, several bars, the huge cinema multiplex and the elaborate video arcade/cafe/bar of X/S Orlando.
Take your pick
(in all honesty, I think the answer is really in what YOU like doing best, as Martin S suggests)