It's true, there is a lot of information. Faster can be better, of course - but is it worth the extra money, is the question.
Unfortunately, there's no way to say "okay, if you're only running Exchange then 667 MHz is fine", because there's always more services that are going to be running in the background, and new things will be installed, and programs will sometimes act funny and consume too much processor time, et cetera.
"Most efficient" is going to be the fastest, but if you are on a limited budget, you don't need the fastest. I think you're on the right track.