I'd agree with what Kibbee said that the 650B is essentially "somewhere in the middle" and it really DEPENDS on the type of riding you do.
I did not test the 650 extensively, but being "better" is a very relative term. The 650 essentially is a compromise, so due to that, it is not better than the other platforms.
HOWEVER, in my opinion, 650 is closer to the 26. I measured the overall diameter of a few 650B tires, most are right at 27 (or even slightly under 27). That's only 0.5 inch difference from 27.5, but 2" smaller than a 29er really.
I made my mind up the first time I demo'ed a 29er. I ride mostly XC and wooded trails. The trails have steep drops and tight corners at the bottom, then steep climbs - essentially a lot of switchbacks along a ridge. Throw in a few flat fast sections, and even some techinal areas, these are my local trails. The 29er climbs MUCH better than my 26. It is overall much faster on my trails than a comparable 26. The only thing I would knock the 29er on is really tight twisty technical turns, but honestly it is not nearly as bad as I was lead to believe (or at least imagined in my mind). I still have my 26, but for my local trails the 29er wins hands down.
For my trails, I personally think 29er is better (i.e. faster, smoother).