If you are custom fabricating something, I would opt for a set of 6.5" components. You've got a much better selection if you do. Not to mention a 6.5" generally will have better transient response than a 6x9".
6 x9's are good for out and out volume but they ain't round so quality = round speakers.
i work in car audio in the uk and generally our cars are so much smaller we rarely fit rear speakers.
depends what you want from them