If you talk to stereo and speaker gurus they will tell you that a round speaker is the cleanest sound out there. Will a 6x9 be louder? Of course, it's bigger. But the sound will not be as clear as a 6.5" speaker. The difference may be so slight that you may not be able to tell the difference, maybe you will. But when doing my research I have a friend who deals with high end equipment for concerts and such and he will only run round speakers. A round speaker has the best sound.
I have a question for ya, have you ever seen an oval speaker at a concert? Go with a high quality 6.5" speaker and amp the hell out of it! I run 6 6.5" Focals in my tower and we all sing along to the music as we board. In fact, it's starting to interfere with our riding. Apparently we're all A.D.D.