Correct me if I'm wrong, but a ff option does bring out more vignetting which isn't the biggest problem in the world, but it can turn a good ef lens on a aps-c into a pain to edit on a ff.
correcting vignetting is a 1 click piece of cake in post...if you even find it unpleasant. for wedding work it can often add to the look of the shot.
The 5dc... some people love it and it is cheap, but I think there is a real challenge in getting really nice photos from that body.
in good light the camera is still really good. in really low light (like what you would find at a ceremony and reception of a wedding) it is almost unusable compared to 5D2. i can't tell you how utterly frustrating it was to try to pull shots off with my 5Dc w/out flash during ceremonies. if you aren't equipped with 1.8 or faster glass you are in store for some serious frustration.....i've been there.
if you really just can't afford the 5D2 then rent it for this wedding. i can't see anyone who shoots weddings and is familiar with both those cameras advising you to go with the classic over the 2.