go the sub. 6x9s are no good i think. if you want speakers on the back your better off with just mid range ones because you get a bit of crackling with 6x9s, compered to mid range or splits.
all i have is the 6" splits in the front and an amp and sub in the boot. it sounds great no need for anything else.
thats what i think anyway
Go with splits and a sub. You won't even be able to compare coax 6x9's to a decent splits + sub combination.
I've currently got jaycar 6.5" splits in my doors and i'm currently integrating a qmax invisabass6 into my cargo tray. The splits sound good on their own, but it needs a sub.