I just got a 21. I'm 6'2" and ride mostly fire road, not real technical single-track. I went with the LBS guy's advice (who has been on 29'ers for 8 yrs): move yourself "further back behind the front wheel" to max out the ride on a 29'er.

I had been testing 19's for the last three months and just assumed that was my size. As soon as I got on a 21 the fit seemed better. Who knows - my inseam, flexibility, frame geometry etc. are probably the final determinants anyway.