I thought UT2K3 wasn't nearly as fast as Q3, but that's my perspective.
Anyway, UT2K4 is identical to 2K3 with the exception of a few new weapons and the Onslaught mode with its vehicles.
I never was much of a fan of 2K3 myself, but I bought 2K4.
Best test of whether you'll like it? The demo. Give it a try.