I'm more an SNG player, but I play NL ring for a couple reasons:

Bonuses - most sites you have to play ring to clear bonuses in a reasonable amount of time, although I am slowly working off a Pokerstars w/SNG since there's no time limit.

Time constraints - don't always have an hour or more to play SNG

Variety - and to work on ring play, though I usually try to do this while working off a bonus which provides an added cushion.

I think you can play either ring or SNGs profitably, but I'd lean more towards whichever you're stronger at then worrying about profit(at least at first)