1 L7 12'' or 1 L5 15''?

 I don't know why people think 10" subs have better SQ than a 15" sub. People need to quit believing every thing that someone else says. A 10" sub isn't any tighter, or responds any quicker than a 15" sub.

The only reason people perceive a 10" as being tighter is because it handles the upper midbass frequencys better. Your subs really should only be playing 60 hz and lower, they don't need to be playing up to 125+ hz like a lot of people have. Having the sub playing so high will cause localization of the sub, meaning you can tell exactly where the sound is coming from.

Anyway... the main reason in moving to a 15" sub besides more displacement, is that they play relatively flat all the way down to 20 hz. The frequency response of 10" and 12" are going to start to roll off a lot sooner than a 15" due to it's ability to move more air when nearing sub-sonic frequencys.