I've been considering the same two camera's for the past 2 weeks. While the F30 takes the cake in the low light department, I wonder just how often I would use that performance. When I take my S2 out (or the old S200), I never think twice about wishing I had higher ISO. I just deal. But, when I take my E550 out, I relish the fact that I can use ISO 200 and 400 without too much noise. I could only imagine what ISO 400 without ANY noise would do for me.
Aside from that fact, I really consider the SD600 a better camera. Now, don't take my word as gospel. I have no experience with either. But, I feel there's a reason the Elph's sell so well.
And it's smaller. I've always wanted my next ultra compact to be more compact than my S200. The F30 is slightly larger, while the SD600 is slightly smaller (thinner, at least).
I would strongly consider the SD700 IS if it weren't so big (in size and in price).
And so it goes...As it was when the F10 was released. I was decisionless for so long that I never bought a replacement. I fear I may do the same thing to myself...again
One of the biggest things holding me back from buying the F30 is that it isn't as small as I wanted. So, if you aren't considering that fact, buy the F30.
P.S. Online pricing shows the F30 slightly higher in price, but if you buy this week you can use the $50 rebate (making it cheaper than the SD600).