Personally I would go with the nVidia. From previous experience nVidia did have a bit of a slip with the series 8xxx and series 9xxx geforces but they've improved a lot since then, but all 4 of the ATI cards I've had in the past have had one or two problems with them, usually in the areas of poor RAM handling or struggling with fast frame changing. Seriously the ATI 4650 HD I bought couldn't even run WoW Cata in 1024x768 without the frame-rate dieing horribly in Stormwind, but then I took that back and swapped it for a Geforce 210 which cost the same price, and that works fine for FFXIV with all settings on medium, apart from a bit of a drop in frame-rate in some overly graphical areas (Black Shroud).

I've found nVidia to be much more game friendly and their nview program just seems to operate better than ATI Catalyst does.