My preference would be for the 5D, size is one consideration, a secondhand 1Ds is likely to have been used by a pro, so well used. Even if it was only in a studio. Really it boils down to your type of photography, if you need or think you are going to need the spec's of the 1Ds then go for that. From the limited reviews I have read and photo's I have seen the 5D is a good camera and would likely be my choice of the two, if I upgraded from my 10D, which I wouldn't mind money permitting. The one reason being the full frame and being able to use my wide angles to the full again!
