In my area finding good 26" mtb tires is getting harder, only the cheep walmart type 26" knobbies are available In stores.

I've had all three wheel sizes and IMHO 27.5" has the best traits of all.
Nothing wrong with 26" except limited tire choices and they feel small under me.
27.5 wheels roll over things real well, like a 29er IMO
27.5" tires everywhere, the good ones...

What I needed and got with the 27.5" wheels:
I can accelerate real fast and go up short punchy climbs better on my 27.5"
I can turn real tight, think switch backs,, with ease on my 27.5"
I can 'Pop' off things better and change direction at speed with less effort and much quicker.

What I lost leaving 29er's :
I have to work a little harder keeping up with 29 er's out In the open on the fire roads,,,
I am not the strongest rider so getting the 29er up to speed took more work but once there they hold speed better... So they are IMO faster overall.

29" wheels and tires everywhere,,, BUT for me they are bigger/heavier by design, the tire and the wheel set. I found that the 29er wheel must be stronger,,more expensive,, or they will flex more, longer spokes,,,,

I'm 5'11" 170 pounds 32" Inseam,
Always felt like I was sitting on top of my 29er
Felt Like I was sitting 'IN' my 27.5"
Felt like I was a clown on a kid trike on my 26", didn't know this until I rode the other wheels I must add.

So what wheel size Is better ?
Your trails and riding style dictate this, as does your size.

I personally think stronger riders will get far more out of a 29er and be able to exploit the 29er's advantages so well that If they can afford good wheels and the newer 29er Geo bikes that would be the best choice for this kind of rider.

Just keep In mind that the middle ground means versatility~~~