please for gods sake don't just go out and buy speakers with the best specs. yes specs are nice, but listen to them 1st, even if you have to drive 4 or 5 hours its always worth it
you are correct that the 500w sub would be 'louder' in spl's i think it is something like every 3 or 6 db, you need twice the wattage.
Dj stevie is right on. Stick with pro stuff that has a proven track record