The 17" has more space for cooling, so it'll be a bit cooler.
The i7 will run a bit more hot than the i5, so the i5 will be a bit cooler.
Important to note: this is only when you're stressing all the cores. Most of the time 2-3 or your cores will be completely asleep doing nothing, and your MacBook will be cool and dead silent, regardless of the size/CPU. Bottom line: don't worry and buy what you need.
I would say this is pretty accurate, but not 100%. Both i7, i5's have a 35w tdp, and while the i7 should operate at a higher Turbo Boost clock, neither stays there very long.
Out of the 5-10 machines Ive seen of each, there was no correlation between heat and i7, or a cooler i5.
Oh and, only 2 cores for each chip
The i5 has better battery life.
here is a link re battery life i was surprised how much better c2d and i5 is than i7.