NVIDIA or ATI Card which is better?

I went shopping for and IMac today, just looking. I have an Apple Store nearby and they let me download SL on a 24" IMac with an NVidia GeForce 7300 GT128 SDRAM Video card. This IMac had 2 GB of RAM. The frame rate looked kinda choppy, the sales man event tried to change the resolution in SL down, still not that great. Then he tried it on a 20" IMac which has a ATI Radeon X1600 128 VRAM card, so much better, smooth as silk. Now both IMacs' graphics cards can be upgraded to the 256 versions, which I would intend to do anyway.

So, does anyone know why this is happening? Does anyone have experience with either of these cards? Do you work on one of these IMacs now?

Of course I want the 24" but with bad graphics, I'm hoping it was just a fluke....

Thanks,
Frank

I just got the 24" w/ the 256 vido card and 2 gb ram. I had to lower the draw sliders a bit from the default settings in graphics , but other than that ive been happy. Some minor jerkinesss, but overall its been great. Much better than i ever imagined. I never knew the water in SL had ripples LOL. Its only been 2 days but so far so good.