There is more than just firmware diff between v2 and v3 MMM. The case is new and they changed some parts on the brainboard. Castle has implied that most of the failures of v1 and v2 should be fixed with the new firmware release.

I bought a v2 and I'm just going to send it back for a v3. The case on the v2 needs modding where the wires for switch enter - case rubs thru wires and they can short out and the inside base of the case rubs the bottom of the board eventually causing probs and internal BEC has continued to be a prob with v2. I would wait.

The 6XL (and 7XL) have a rep for running hot so with better cooling on the MMM thats the one I'd go with too.