In the beginning, it doesn't really matter. Belt is cheaper to fix. Belt can be the cause of problems initially. For me, being new to building my own, I didn't tighten the belt or check it after a few flights, and it slipped, causing a crash. It did even on my Trex 450 Sport, and you'd think I would have learned my lesson after 3 clones...

Shaft should be better, but the only heli I'm having tail problems with is a Trex 500 ESP where I get tail kicks which I don't understand and haven't been able to correct. Only at rotor high speed on idle up mode...