The answer might be "neither." If you don't have good temperature control for fermentation, I'd get that set up first. Whether your wort come from extract or all grain, and no matter how you package your beer, you'll get the most bang for your buck by getting fermentation temperatures under control. A decent used fridge or freezer and a temperature controller, and you're on your way.
For me, the biggest jumps in the quality of the beers I make came from proper pitching of yeast and good fermentation temperature control. If your primary goal is to make good beer (I think it should be), good fermentation practice is the key. Wort production and packaging are really secondary.
All that said, I'd probably brew a lot less if I hadn't switched to kegging several years ago. And, as others have said, jumping to all grain is really inexpensive if you're already able to boil and chill a full-volume batch.