I had the i7 for about 10 days after having an i5, and I noticed the fan came on more, but I really didn't do much that would have highlighted a difference in battery life.
I returned the i7 for the i5, the slight increase in performance wasn't worth the extra heat in my mind, but that's just me and I'm pretty anal about fan noise. Its not that it was running all the time... it just would come on more frequently and sooner than for the i5. I don't regret returning it. I wanted the fastest I could get... but it just wasn't worth the extra heat (for me).
You can't go wrong either way, its a GREAT computer!