That would depend on your objectives.
Two pair of XM7s would give you more peak output. Keep in mind that you are now developing a little more than a 50 percent power increase in addition to the obvious increase in the collective surface area with a second pair. More quantity of the same speaker increases output but has a minimal impact on the speaker characteristics.
The XM9 will have deeper midbass extension and a warmer balance to begin with.
So based on my personal objectives, and given the choice between just these two options, I would go with a single pair of the larger XM9s.
David
Earmark Marine