Since I can only afford one bike and am and AM/DH rider who would love it if there were some local enduro races...I'm with 26ers. For a long travel bike, they rock. If I got a hardtail I would probably get a 29er though. I've rode a few and really liked em.
I ride 26" for downhill, and the 700c (29") for the road of course. In my humble opinion, 26" makes sense on off-road stuff, maybe 650b would as well, but I don't see me loosing 200mm suspension upfront to fit the 650b. For road, there are no questions really, 700c is the way to go. 26" even looks odd on road bike.