iv got new 5x7s in the front doors and in the back i can put 6x8s but i was jsut going to cut the hole bigger and put in 6x9s. but then i found adapter plates to put in a 6 1/2'' in the back instead. what would give me better sound.
6x9's might be able to handle more power BUT they will start to distort easier than round speakers. odd shaped speakers (oval, square) have stress points at certain spots causing distortion if enough power is fed to them.
On a good note, the 6x9's can probably handle lower frequencies.