Depends on what lenses you already have for your Canon body. If you have a selection of lenses suitable for the FF 5D, then that's the obvious choice! If you'll be basically starting from scratch lenswise it gets a bit more complicated, although I would still recommend the 5D to YOU because lenses suitable for the 5D will work perfectly well with your current body (if you're keeping it).
Quite apart from the brand issue, the 2 cameras are very different animals and can't really be directly compared. The D2x is a full pro spec Dx body with a 1.5x crop factor while the 5D is a prosumer full frame body. Personally, I reckon it also depends what use it'll be put to - if your tsates lie more towards wide angles, FF is probably what you want, while for telephoto (wildlife from a distance for example), the 1.5x crop factor of a Dx body will give you extra apparent reach (although the same effect is easily achieved from an FF body by cropping to suit!).
Thinking a little more, FF every time IMO! The 5d will be newer than the D2 in terms of technology so will almost certainly handle higher ISOs and lower light better. If possible, try the 2 options side by side and take a few as close to identical shots on one of your cards to compare at home. Since you already have a Canon body, you'll be more familiar with their navigation system than the Nikon's too.