You won't even see a difference in battery life. There are so many people buying SSDs and installing them just to get about 2-4% increase in battery life.
Other than that, performance, you won't even notice it.
in this case, the OP is considering going to a slower drive. I wasn't suggesting that the 7200 would eat more juice
it's conceivable that the slower, larger drive's increased seeking would noticeably reduce his battery life. that said, there is also the consideration that he'll have a faster drive with more usable space with the 5400.
a 750gb 5400 drive likely has a faster usable disk area than the 100gb 7200
i'm not up on my terminology but the gist is that the 750 should have a few hundred really fast gigs compared to the few dozen really fast gigs of the 100gb.