I too have noticed that 64bit builds use a hair more cpu, though I've never seen as much of a discrepancy as DrewMelton. I have noticed that 64bit builds use more memory than 32bit builds though, which seems kind of ironic.
That being said, I have tried newer NW's, but always end up rolling back to 10.5. It's is the smoothest and most stable for me, and everything works that I need...but 10.5 only exports 32 bit. So, that simplifies things.