Most cases, I take the key of the woman cause they usually sing the melody and I get the key from that.
Interesting. I'd think of the key being more of a fit for the man.
The problem is register. For example, most men can't sing in an upper register, like an Israel Houghton or a Micah Stampley. Women, would have an easier time singing the lead to, say, 'I am not forgotten' in it's original key. Men, on the other hand would need a lower key.
So, when you're singing a song like, say, 'When I think about the Lord', I'd think about finding a comfortable key for the man (after all, that particular song modulates once, anyway).
I'm out.