I would agree with sushi's comments entirely and add to them.
If i were to setup a smaller HT system in which the primary use was HT and not music, i would be inclined to stick to the smaller 'satellite' or bookshelf type speakers for its design. In this instance, i would certainly use the vertical centre, again considering the components involved in the HT.
For example, if i had a small room (15x15ft), with an entertainment cabinet and a 40 inch TV, i would probably consider something like the M22 or M2s as mains, add a sub, QSx for surrounds and an M2 for the centre.
However, if my room were 18x25, i might change those main speakers to M60s so the larger sound and volume can handle the larger room size.
If i had that same size room but a front projection with an 8 foot screen, the little M2 as a centre channel could easily be lost against the large screen. I would be considering a much larger centre for such a purpose and if i stayed with a vertical speaker, then depending on the screen heights, an M22 or an M60 may not work out so well in the center of the room.
You would probably want to look at the horizontal centres to provide a wider sound dispersion in this instance.
In my present situation, i have a 50" RPTV and placing an M2 or M22 on top of the tv as the centre channel draws the vocals up way too high for my liking. The extra width of the VP150 also creates a wider soundstage across the centre in the front so i can move my M60s out further apart, again to fill a larger space.
