first of all, rewrite your question, i don't understand it(is it- will 1 15" be better then 2 12"s???) and also, will it be better in what? SPL or SQL, if you want the better all around sub it's the JL, but if you want whats louder, probably the l7
all things being equal (very important) generally, a 15 will get lower than a 12. 12s are quicker and more punchy than 15s". it's a matter of preference. i like them both, but i prefer 12s. of course, available space is an issue as well. the L7 vs the w6 however is not preference. the jl eats the kicker up all day. that is, its a far superior sub.