I would say that depends on your listening style. and how picky you are with your sound. i have two mtx 8's right now.(would prefer jbl but they still sound good) they actually do really good. even got some punch to them(with 1600w rms,i wouldnt generally recomend that but i dont care bout them to much haha) but they sound good with alot less power as well. but with my wide range of music i listen to i occationally am bugged because of notes that 8s just cant hit.
10's seem to be better for all around listening but iv noticed if your running low power you may not get as much punch as rumble. but with enough power they can punch dang good as well.
just my 2 cents with what ive seen in my own systems.
oh ya and 8s in the wrong box and not much power can make ya feel like you didnt do anything but waste money for hardly any sound added.