I have a USB External I use for my iBook. It is a pain because you hve to plug the transmitter into it, but i can't say i've noticed any battery loss with it. I would GUESS that bluetooth would use more battery than USB.
Mine is a Microsoft mini thing - 2 buttons and a scroll wheel. Up until yesterday i LOVED IT. However, yesterday my old Apple one button mouse was replaced with a Mighty Mouse - oh my, i love that thing! I was reading about people not liking the wheel ball - that is the perfect size. Don't even have to move my finger just wheel!
Anyway, i'd suggest a trip to Office Depot/Staples/_____ and try how they feel. But *I* would gladly go steal that wired Mighty and put it on the iBook 😀
(On the MS mouse - the really nice thing is the USB plug in part snaps on the bottom of the mouse and it's easy to carry around!