The LCCs tend to have better economy class than the legacies do for non-premium economy for non-elites. I'd rather fly Southwest than standard economy as a non-elite on United for a short notice business trip, since a short notice booking on United usually means getting an undesirable seat if you are not elite. Plus, Southwest has a bit more legroom than United's standard economy.

Which brings up the question for the original poster: how much is "a lot" of flying? Specifically, compare the expected amount of flying to the elite qualification levels and elite perks of various airlines and take that into consideration (as well as which airlines fly where you want to go) when choosing FF programs. Elite perks (like using the empty first class checkin desk or United's Economy Plus seats with more legroom) can make a significant difference in how pleasant a legacy airline flight is.