Two 10" will be fine,Two 12" will be even better hits hard etc....BUT don't plan on grocery shopping with your IS cuz ain't gonna be room for shiet...plus its heavy.....I just sporting One 12" eclipse. Better then 10" Not as good as Two 12" but just enough....Even One 12" in small q-logic box is taking up tooooooooooooo much space.....up to you though
i have two 10's jl subs with a jl amp and the bass knocks pretty hard. i think 2 10's are cool but a bigger size would be better but more risky since it leaves no room in the trunk and it would be a bitch if you need to get ya spare tire out. if you want monster bass you should go for the jl w7 sub.
I would go with 2 12s, simply because the 10s lack the depth that the 12s are capable of. There are some 12s that can work VERY well in a box the size of a large box for a 10" speaker, just keep that in mind when buying the speakers (if your worried about size and weight)
If you (for some reason) wanted to go with a pair of 10s, you can design a pretty good ported box to really thump out the low notes, but box size will get back up there in doing so. (I'd get the 12s )