There are still people that buy the 320/640 because they don't know any better.
Believe it or not I've seen one or two people still recommend the 640MB version of the GTS over the new 512MB. Obviously they don't understand that one is G80 and G92... plus people think more money = better product.... Oh well.
The GTS is a better performer (beating the GTX in some benchmarks), but the GT is better performance per dollar spent. If you're on a tight budget - GT, if not - GTS.