between various message boards?

I realize that this could vary greatly depending on mods, skins and what not. But if you were talking about stock boards... how can someone know which forum software/script will be the lightest load on the server?

This has always been a puzzler for me and I thought I would throw this out and see if anyone had any ideas about how you would get a feel for this without having first hand experience with a large vBulletin board, large IPB board, etc etc.