with mtm the forward lobe is a little narrower than tm, so it becomes a little more important to be pretty much on axis (vertically). in a tm the beam can be tilted up a bit too giving a little bit better sound when you "stand up". not huge differences though, ideally outside (or at least at the boundaries) of the horn anyway.
I don't know what to make of the latest karma 15 that uses a guitar speaker with no overhang (no xmax) if I read the sheet right. seems like it could work well if the bass is really kept out it, so the cone isn't moving very much.