And THIS.

I'm riding a 650b right now.  It is very good. Very Fun.   Here's what I'm doing for my wife's new bike because she hates her 29er....you could do it too.

Get this nemesis sport.  Do raceface aluminum cranks, raceface narrow/wide ring, x9 type 2 rear derailleur, light bars/stem/post, carbon tapered fork, xo gripshift, crest wheels.    Be $1500, superlight.  

You can talk all you want about the weight, rolling resistance, acceleration....but the 650b just feels right.  If you put a 650b with 2.1" tire next to you 700c wheel with tire, it's about the same.  It makes 29er bikes feel over bearing, fat, and stupid.