That IS the only difference.So if they all have the ability to play the same. What exactly is the difference between an 18" and a 10" besides the fact of the cone being bigger????
Since you're doing a wall and should have plenty of room I'd do a 10, 12, 15, and 18 to cover all the freqs. Should be very accurate and hit the lows nicely also. Might even consider running 4 diff brands of subs for that extra pimp look....