After trying almost all sizes between 32 & 42mm, I've settled on 38mm Schwalbe Marathons on my Trek 520 and 40mm on my Haro 'Niner. I use the 520 for centuries and touring & the 'Niner more as an around town utility/commuter/winter bike. I found 33 and 35 too narrow on the 520 as I had too many snakebite flats with them on the poor road surfaces we have here in Upstate NY. The 38mm & up seem sufficiently robust as I haven't had any such flats since using them. Perhaps if you're riding on better quality roads than I, the narrower tires would be OK. At one point I had 42mm Conti Top Contacts on my 520 and then switched to the 35mm Schwalbe Marathons. I saw a huge decrease in rolling resistance which took about 7% off my average times. Of course some of that might also be due to inherent differences between the Schwalbes and the Contis regardless of size. I then switched up to the Marathons in a 38mm because of the aforementioned snakebite flats. The difference in rolling resistance between the 35 & 38 mm Schwalbes seemed to be negligible and I've had no snakebite flats since. Just my $0.02, YMMV.