Which subwoofers are better?

I have 2 12" Memphis pr 250 Rms a piece 500 max.I'm using bazooka 500.1 amp there is a 15" Cvx kicker in pawnshop for 100$ in a ported box should I trade and get one 15" Cvx or keep my 2 Memphis 12. Which will have more b what's the rms difference.also I have a 96 Saturn so space is little but they will fit