Actually, no, it's not such a no-brainer. There's been threads here (among the 9001 threads asking the same thing - i5 or i7) where people have reported the i7 running cooler under load than the i5, AND having slightly longer battery life. Though at idle, the i5 is the winner (though not by much).
So, as always, it depends on what you're doing. But it appears that the i7, while it will use slightly more energy and run slightly hotter if you're not doing a whole lot, can do more things more efficiently when made to work hard.