Personal preference I reckon Dave...
I like a bit of rear fill, keeps the kids happy My setup basically: 12" Sub, 6.5" Splits in the doors. Both powered by the amp, 6.5" Coaxials in the parcel shelf powered by the deck with the fader slightly to the front...
I reckon the 6 x 9's would make a mess of the sound, but ultimately be louder... In your situation, I'd either go with the 4's or fork out for some cheapish 6.5 coax's...