I would say try out the 1x12 3/4 open back first, if you haven't already, and do a side by side comparison to a closed back cab.
When I tested the Mk V in the store, they had it plugged into a Rectifier 2x12 closed back cab. This is a matter of personal taste of course, but I think the closed back sounded a bit better -- it had a certain "chunk" to the notes that the open back does not seem to have (and I realize that I am comparing a 1x12 vs a 2x12).
Don't get me wrong -- I have both a 1x12 and a 2x12 3/4 back now at home, and the Mk V sounds awesome through either, but after hearing the closed back in the store I went so far as to put some leftover Auralex foam on the bottom and sides of the 1x12 and it seemed to tighten it up just a bit.
So I guess that, ideally, I would like to get a 2x12 closed back so that I have the best of both worlds. But check out the forums, there have been several recommendations on cab choices (even non-Mesa ones).
If you get anything, please let us know how it turned out.