I prefer out the sides - it sounds better. Even on my truck (Ram horn manifolds, 2 into 1, 2 1/2" pipe) it sounds nice and throaty coming out the side. I think you lose too much going out the back, and it looks sorta funky, too.
I've been contemplating a smokestack set-up lately.. dual pipes coming up behind the cab in a semi-truck fashion. I think it'd be cool..