There is barely a difference in sound between the Mark IV and the Mark V... the V offers a few new voice options, but isn't any heavier, for example, than the Mark IV. It is, if anything, more like a Triaxis with a variable wattage power section. You're not going to get much different tone from a Mark V than from a IV, and thus not much different from a Triaxis if you're using it with the 2:Ninety power amp.
It's cool and it's new. If you didn't have a Triaxis rig or a Mark IV, it would be worth considering. But if you have a IV or a Triaxis rig, you're just not going to experience enough of a difference in your tone to make it worth the purchase.
Comparing a Triaxis rig with a Mark IV/V is sort of a silly proposition. The question then is really more about if you want to give up a nice, stereo, rack rig with memory for a ton of different amp sounds for a head. And unfortunately, due to the size of the new circuitry, it's not possible for Boogie to build a short head that can fit in a 19" rack, like they had with the Mark IV.
Mark V? I'd take it instead of a IV. But I wouldn't give up my Triaxis rig for it.
Scott