The size difference between a true 5x7 and true 6x8 is so minute that most people don't even consider it and call them "identical". In reality, most manufacturers don't even change the size of the cones between the two models and instead make up the size difference in the mounting ring.
That said, I have a pair of Kenwood 6806C 3-way 6x8's in my rear deck and a pair of Kicker K57.2 5x7 components in my doors. The Kenwoods give better bass but the Kickers have a more natural sounding midrange, IMO.
I most likely will be replacing my 5x7 component set with a 6.5" component set in the near future and using a homemade masonite adapter plate as described above.