Never used Priceline, but I know it used to be that sometimes you could get really great deals on flights the airlines were trying to fill. I don't think you'll find that now. The airlines have cut a number of flights and it's rare you see a flight that isn't completely full. Especially between two large cities.
So, you're better off buying well in advance. Prices will be better, and you'll have a better selection. There are many travel sites, I've never found much difference between them really. I tend toward travelocity or kayak.com