Agree with the others, it’s all about the kind of riding and/or racing you’ll be doing. I think for many XC racers 29ers are the way to go but if you’re riding for fun at trail centres the added suspension travel of 26″ bikes may have the edge in terms of the entertainment stakes.
I’d always read it was the 26″ bikes that were better in tighter situations but actually found my 29er (Canyon CF SLX) to be superior to my 26 full suspension bike (Zesty 514). It’s very nimble and responsive in the tight stuff with great balance.
I think it was Mountain Bike UK last month that ran a comparison between 26, 27.5 and 29 bikes. The 29″ bikes were the fastest over their test course with the 27.5″ bikes a close second. The 26″ were noticeably slower. As with all these decisions it’s best to test ride the bike in question on the terrain you plan to ride.