Basic Speaker Upgrade - 5x7 or 6x8

I want to upgrade my factory speakers (originally a MACH 460 system) in my 04GT. I have head unit, 4 channel amp, sub, and sub amp. I simply want to drop in some speakers in the original door and rear deck locations. I do not want to use component speakers. Simple question - which fits best - 5x7 or 6x8 - Since the car may be sold in a year or so, I don't want to do any cutting or fabricating, I just want to drop them in. Are there issues with depth on the door speakers?

I read the FAQ sticky and some great info, I'm just looking for the simplest info here - what fits best where - doors and rear deck.

Thanks!
-T

Thanks Guys I went ahead and got the 5x7s.