I recommend .30's and .28's if money is an issue. I find there is a bigger jump in pricing between .30's and .28's.
Here's a good read for you:
http://mackila.com/airsoft/atp/07-b-08.htm
In theory, flight time should be lower with a .30 at typical outdoor ranges.
In any case, I recommend that you prioritize quality over the available weight. For example, I would use .25 BB Bastards over .30 crap brand any day.