I ride a 2015 650b.. The main thing that is forgotten in the review of the two is that the 650b came with a 34 tooth chain ring and the 29er came with a 30. This makes a major difference!
The first thing I did was change to a 30 tooth, and good to go. It climbs well, and descends like a beast.
I have the '14 29er Comp. I can't compare because I haven't ridden the 650b, but to me it seems counterintuitive that the 29er would climb better. In theory, you have to spin the cranks more for the same amount of wheel turn on a 29er compared to a 650b.