I play them both about 50/50, but I play NL at very low buy-ins and FL at middle stakes, so my play at NL won't cripple my bankroll and at the FL I don't really need to worry to get much punished.

If it were other way around I would stress my ass off at the NL and would be totally bored at the FL.

Which is more profitable? Depends on the person and his luck.