I ride 650B's on one bike, and 700x37 on the other.
The 700x37 Continental Top Touring tires I have on my commuter/touring bike are amazing. I run them at a range of pressures from 40psi up to 75psi. At low pressure they are comfortable on rough fire roads and trails. At high pressure they roll nice and fast on the road, but are still very comfortable. I'd say the wheels and tires are every bit as durable as the 26" wheels I used to ride on my commuter that was converted from a mountain bike... only these 700x37s have less rolling resistance than a typical 26" mountain slick.
My wife's bike has 650B wheels with Panaracer Col de la Vie 36mm (or are they 38?) tires. These things are comfy! They can be ridden between 25 and 50 psi. I've been riding her bike a bit lately for a change (good thing we are the same size!), and 650b are just fantastic. If you're torn between 26" and 700c tires, 650B might feel like the best of both worlds.