Takedown:
I have the ATI Radeon 32MB DDR in one of my machines, and a GeForce2 GTS Pro 32MB DDR in another of my machines. At the risk of touching off an endless debate, I have to say: It's tough to spot a "real-world" difference. The GF2GTS Pro regularly benchmarks higher than the Radeon, but in playing Quake III, Half-Life, Unreal Tournament and others, I can't tell any difference. (I actually spent considerable time LOOKING for a difference, too, just because I was interested in this debate.)
(One thing: When I upgraded to DirectX 8.0, my GeForce completely freaked out. I tried every combination of new detonator drivers, even the leaked v7 ones, and I still had problems. Ended up having to wipe DirectX. Really, I don't recommend upgrading to DirectX8 right now anyway, largely because the uninstall is hell. On my Radeon machine, which accepted the upgrade fine, I haven't noticed ANY performance increase.)
By the way: The "Pro" version of the GeForce2 GTS is a special OEM version (Gateway and Dell sell it, among others). The truth is it's more marketing gimmick than anything. They open up the pipeline wider, but you're not going to notice a performance difference over the "regular" GeForce2 GTS.
My advice: Pick the cheaper option. At the time I got my Radeon, it was far cheaper than a DDR GeForce2 GTS. Now, that's not so true.