I would second the FS choice. I ride a 650b (27.5) hardtail and I enjoy it for my solo riding - small, light, snappy, and I can throw it around with much more agility. But it's just slow compared to a bigger wheel FS bike... in the few XC style races I've done, I just get frustrated at much faster the 29" FS bikes descend, cruise over rough terrain or just roll on the flats. I got to demo a Giant Anthem (full on race bike) carbon FS 29" and boy... that thing was freakin' fast!!!
__________________
"Your beauty is an aeroplane;
so high, my heart cannot bear the strain." -A.C. Jobim, Triste