Okay, so I am putting together a system for SPL competition. I was wondering, which would work better, a 15" or an 18" subwoofer? Let's assume they are both the same brand, like Sundown Audio, and in the same sub lineup. I just want to know in general, which would be better? Let's assume that I have the space to do either sub in my vehicle, are there general differences for SPL to consider? Or is it one of those things that really vary widely depending on factors, and if so, what factors?