I am leaning towards lower ISO's higher quality files, I have a 5 Mark II, its nice does decent up 1600, 3200 requires careful attention to exposure but it leaves me wanting a better ISO 100. Landscapes and portraits are my bigger concern. Color rendition is important also.
Thank you to the two people that have taken the time to give thoughtful and detailed responses.
Ok, I see your situation now. If you keep your 5D Mark II, you'll notice a drastic, and very noticeable improvement in color rendition, accuracy, saturation, at ISO 50-400 in the 1Ds3 over the 5D2. I shoot the 1Ds3 in Portrait mode, and I just absolutely love it. It's pretty magical in those ISO ranges and I have not observed any banding in the shadows yet. So yeah, you'd have your range covered with the 1Ds3/5D2 combo.
The other thing is the metering accuracy of the 1Ds3 over the 5D2. When I import RAW files the camera got it exactly right, whereas I cannot say this for when I owned the 5D2.