If you're a developed enough rider that you actually recognize a practical difference in your ride when you choose a wheel size that's suited to your style, then you should do that. Most of us who are still learning or riding recreationally will do just fine on a bike of solid quality, regardless of wheel size.
I started out on a borrowed 26er hardtail, and had so much fun I was ready to spend money even though I was hopeless on the climbs and clearly had no idea how to descend (flats were fairly okay). I tested a top-of-range full-suspension 29er for a full day on the trails, and my perception was that it was a bulldozer: unstoppable, but not what you'd call nimble. I had less fun than when I was (in my own mind) skilfully piloting the 26er through winding lines around obstacles rather than just plowing over them.
I ended up buying a full-suspension 26er, and it is very obviously more bike than I am rider; I haven't yet managed to ask it to do anything it wasn't capable of, but it has definitely asked me to do numerous things I wasn't capable of (climb that giant scree slope! ride off the edge of this four-foot drop! don't fall on your face descending this slick rock face!) At no point have I thought "I could do this bit if I'd just bought that 29er". Plus I'm really enjoying buying up all the cheap 26er parts (especially tires) that the fad followers are feverishly disposing of.