If I were you, I would let the brand of speaker you want dictate what you get, rather than the size. For instance, if you really want Infinity speakers and they come in both a 5x7 or 6.5 inch size, you probably would not be able to tell the difference, it will just be easier to install the oval (stock) size. BUT, if say you want Alpine (or some other high end brand) that only comes in a 6.5 inch (or other round size), I would just get those and either buy or make an adapter plate. A better speaker will always sound better, whether it is oval or round.
So in summary, I would go first with what kind of sound (speaker) you like, then look at what sizes are available and get the closest to what you need. Components are great, again regardless of what size the woofer is. Are you using a sub or amp(s)?