"If you were going to have only one body for everything from BIF to landscape to macro, which one?"

It depends on the proportion. I do both/all. I chose the 5DII. But then I have a 1DII.

"If you were going to have two bodies, which ones?"

1DIII + 5DII if you have the money. 50D and 5DII if money is a factor.

On your other question, the 5DII has wonderful low light/high iso performance.