I have an 29" LaPierre Hardtail. What is the difference between 29" wheels and smaller ones?
I'd like to know this not only for single trails but also on the road and / or downhill trails.
Pretty much been done to death with evangelistic enthusiasm on many forums.
Smaller wheels - more agile and responsive, lighter/stronger, easier to control
Larger Wheels - roll better over small obstacles therefore considered faster in straight lines, lower tire pressures (due bigger tire) provide more traction off paved roads. Slower to accelerate (debatable how much the difference really is) due more rolling weight.
Don't for 650's - between 26" and 29"
I grew up on 26", and switched to 29er a few years ago, my wife recently got a 650B. On the road the 29er is better, on tight, technical (i.e. have to slow down significantly into corners then accelerate) the 26" has it all over the 29'er in every way. My current preference is my wife's 650B as it suits my current riding best, but bike geometry probably plays as big a part.
You height will play a part - I would not recommend a 29er for someone under about 165cm.