Best tip, as always, is to simply test both out before you settle on which mode you like best with your content and in your room. I've heard quite a few demos on both width and height and in each I've personally prefered height, as it felt as though the 'space' increased to make walls and ceiling "disappear" - but it would very much depend on your own preferences and the room in question. No one can really tell you which mode is better than the other.
If you can spend time experimenting I'd simply recommend that. Height-speakers might be a bit tricky to test without permanently installing something but if you can get around that then do it. A friend of mine moved two tall bookcases and placed them on either side of the tv-cabinet then his height-speakers ontop of those just to test. He finally wall-mounted them.
Since your Onkyo is a 9-channel amp perhaps you have also considered running both height and width, unless the last two amps are already taken? I've heard alot of people say thats more effective than an additional pair of (back) surrounds.