I weigh 220 and I ran 23s all last year, until about a month ago. No flats at all, but the ride was bone-jarring at times, and my bike is Al/CrMo (read: pretty smooth ride).

I switched to 25s last month, and on my first real ride out on them (took a test ride after putting them on the bike to see how they felt), I got a flat about 3 miles from home. Fixed it, went on my way. The ride is noticeably smoother. I ride mine at about 115psi, as opposed to the 130-140 I had to use in the 23s.

As far as speed goes, it seems the 25s are faster than the 23s for me. The 23s I had were skinny 23s, and deflected quite a lot even with 140psi in them. The 25s I have seem to be fat 25s (they almost scrape my fork crown) and barely deflect at all at 115psi. Maybe that has something to do with it. At any rate, unless every last tenth of a mph counts, you get the same workout either way. I say, big guys = big(ish) tires, for comfort's sake. Your arms and back (and arse) will thank you. Mine do.