The only difference I noticed when riding the different sizes is that 29ers lose less speed over difficult terrain (especially on a DH course).
They are better at cornering (even on tight switch backs)
Much more traction, especially in the corners.
Smoother acceleration.
Both I and Britt ride a lot better on long travel 29ers (she is only 5'6) as we have more grip in the corners.
26ers are more fun in the air as they are easier to do tricks on.
650b just seems to be goldilocks marketing BS, basically a more saleable 26" bike. There really is not much of a difference.
As for DH riding (which most articles don't even cover even though there are some good 29er DH bikes out there) 29ers excel.
Due to my rear shock getting damaged in storage I had to rent a bike for the second half of my holiday in the alps. The only one that fit me was the bike shop owners bike, so not a worn out rental vs a good condition bike.
So got a good side by side compare between my WFO and a custom Kona Coilair.
Mine is an enduro bike, wheras the Kona is full on DH.
The main difference was how slow the Kona was over choppy terrain. The niner flew over stuff that slowed down the Kona. The difference was so significant that I have an easy example. As the Kona was slow it lost speed on it's own, the only time the niner slowed was when I hit the brakes. The front brake on the Niner ignited 2/3 down a 2000m decent (Formula the One brake, sintered, shimano 203 icetech disk). I spotted smoke billowing from the front brake, when I stopped flames came out. When I did the same decent on the next day (I swapped the brakes over as I prefered mine to the demo bike), at the bottom of the 2000m the disk spider was hot but touchable, so about 60-70 degs.
As for handling, the bikes both excelled on the fast technical switchbacks, the Kona had the edge on rear travel (6" of pedal friendly travel compared to 8" of DH isn't fair).
29ers are just better unless you like doing tricks.
The article is spot on though, you will be disappointed if you have floppy wheels or a floppy fork on the 29" bike. The additional torque is significant.
