I have a 5D mk1 and a 1D mk2. They are both great cameras but in different ways. IQ obviously the 5D beats the 1D and tbh I've looked at some 5Dmk2 images and I prefer the Mk1 images (but I may be the weirdo). Durability the 1D beats the 5D, telling you something you already know.
If I had to lose one which would it be?
I think it would be the 1D I'd get rid of, the IQ on the 5D just has a quality/feel to its images that words cant explain.
The 5D doesnt do night photography all that well (not sure about the Mk2, but I'm guessing its the same given it has the same AF etc), but I hear sticking an ST-E2 on top helps the AF a lot and doesnt "flash" as such.
The 1D does everything well but just lacks that something the 5D has in terms of IQ, maybe its just the 1D is a really solid reliable performer whereas the 5D can be a prima-donna but does when working well just blow you away.
Different markets I suppose as far as Canon were concerned.
Personally I think if you get the 5D you wont be dissapointed, if you get the 1D then maybe you will still be thinking "should I have gone FF".
No help I suspect.
Matt