I'll assume you're asking about either the 5D mark 2 or 3 & not the original 5D. I've been shooting a 1Ds2 for the last 5 years & I had a 5D when I bought the 1Ds2 and I bought a 5D2 when it first came out but sold it about 4 months later. IQ of the 5D2 & 1Ds2 is extremely close & someone once posted that he felt you'd need to print at the size of a living room wall to see a difference, which I agree with. I've stuck with the 1Ds2 because of the better AF which I need for action, & although I only tried it once it seemed adaquate in low light but for your intended use I think the 5D2 or 3 will be the better camera. The 1Ds2 can shoot decently at ISO 1600 but most prefer to stay at or under ISO 800 with it. My testing of the 5D2 shooting ISO 3200 was (in my opinion) as good as the 1Ds2 did at ISO 800, & from everything I've read the 5D3 can do ISO 6400 and deliver a similar photo. AF of the 5D3 is considered to be better than any 1 series body except the new 1Dx.
Based on everything I've read, and I've done a lot of research on the D800, 1D mark 4 & the 5D3 I'd recommend the 5D3 as the right Canon body without breaking the bank followed by the 5D2 if you need to save a bit for lenses.
"A skill is developed through constant practice with a passion to improve, not bought."
