I just picked up the sample issue of ACA's "Adventure Cyclist" magazine at REI last night. It has an article entitled "Wheel Size Matters." They concluded that 700C wheels are best for narrow tires up to about 30 mm, and the smaller 650B wheels are better for wider tires, 30 to 42 mm.
Would that article have been written by Jan Heine, aka the lead advocate for 650B? Either way, I'd view that recommendation with suspicion. I've seen a total of two 650B bikes. They're not common. Tires for them are even less common -- I've only ever seen them available through the web / mail order. I'm leading up to this: if you're going to ride 650B, take a spare tire, or, if/when you have a problem, be prepared to wait (over a long holiday weekend?) for a replacement to get to you.
700C wheels and tires are everywhere. So are 26". You can get 700C tires from ultra-skinny to way wide; 26" you can get only from pretty skinny to pretty wide. If something goes wrong on the road, you can replace either at the next bike shop. Maybe not with your preferred width or tread, but if something happens you can get back on the road. There's a lot to be said for standard parts, and 650B is anything but standard.