I weigh about 240 without books and laptop, and I gave up on tires smaller than 32mm (label size; most are narrower in real life) years ago. I have 35s (Paselas, which run true to label) on the Atlantis and Rivendell Ruffy Tuffys (27mm actual) on the Rambouillet, and I don't plan to buy anything smaller ever again.
The Rambo is fairly new, but I've had the Atlantis almost five years and used tires from 25-41mm, so I can compare. On my normal rides and 12-mile commute, I'm not any slower on the 35s than I was on tires almost half an inch narrower, and the lower pressure (75-80 psi vs. 100-105) is much more comfortable and stable. The two bikes feel very similar (the Rambo's a little lighter and about a degree steeper in the angles), but more and more, when I just want to hop on one and go for a ride, I pick the one with the fat tires.
FWIW, I've dealt with Rivendell since 2000 or 2001 on several things, including trying to build up the Atlantis as cheaply as possible with parts I already had (had two kids in school at the time and no extra money lying around), and they've been helpful and knowledgeable. I was skeptical of some of Grant's ideas at first, but everything I've tried has worked.