I'd say GT, i have Chaintech 6800GT and I'm happy with it.
I was ablew to play FarCry with HDR @ my LCD's native 1280x1024 and 8xAF and frame rates were smooth enough 30+. Same goes for Splinter Cell demo.
Chronicles of riddick on the other hand have problems with 2.0++ path, as speed drops to 20FPS or so, but then again XL will newer run this part at all and afaik looses in 2.0 path too. Anyway I finished game first time on 2.0++ path and it was OK, and looked cool with soft shadows, now I decided to play on 2.0 path and it flies
Moreover I've seen cases where XL can't run some shaders because of hardware limitation (relief mapping) whereas GT can easily manage 80+FPS
Concerning AF and AA quality, AF was way higher on nvidia till FX series I think, then they sacrifised it because of frames per second battle.
Current implementations are pretty much equal and I have a feeling that otimizations might have been left on on NVIDIA if AF tests shows big difference.
BTW, are AF optimizations switchable on XL (color mip lavel stuff) now?
I'd recomend to be cautios with chaintech though, when i brought my card it was advertised as 350/1000 card, yet it turned out to be 350/900 card.
When I contected their tech-support, they did nothing but changed specs on their homepage, without even replying me... No more Chaintech for me...
Core 2 Duo E6300, MSI P45 NEO-F, Club 3D GTX 260, 4Gb DDR2-800Mhz, Audigy X-Fi Fatal1ty Champ1on ed., 0.5Tb+1Tb Seagate Barracuda 7200.12, 630W AXP, Samsung SyncMaster BX2450, ViewSonic VP171b