Single 18 in a trunk car vs 2 12's

[iceman1575]

I've been debating on a new build for my Grand Prix. My original plan was to get two HDC3 12's in separate chambers, with the ports firing into the cabin, subs firing back. I had an idea to run a single HDC3 18 in a trapezoidal box (36" wide with a sloped back to fit the 18" sub in a 16" high trunk, either sealed or with two ports coming from the sides firing out the middle into the cabin. Does the plan sound good, or would I be better off with two 12's, or even two 15's?

P.S. Sorry for all the posts mods, trying to decide what would be the best.