Ever since hearing Mattayus' one at the UK meet I've been intrieged by the Dave Mustaine cab, and how different it is to a regular 1960. And to be honest, I don't think there's much difference at all.
Dimensionally, the Mustaine is a hair bigger. I'm guessing it's in the height (since Marshall handily don't tell which dimension is which), but one dimension is 765mm on the 1960 and it's 770mm on the Mustaine. 5mm (less than a 1/4") is going to make fuck all noticeable difference to the tone IMHO.
Next, there's the weight. The Mustaine cab clocks in 6.7kg heavier than a regular 1960. The speakers (more on those below) account for 4.3kg of that (I emailed Eminence for the weight of the 12AX75), so I'd say it's a reasonable assumption that the metal grille accounts for the remainder of that.
Finally there's the speakers. 1960's ship with Eminence 12AX75's, Mustaine's V30's. This is where the big tonal difference will come from, end of story.
So, to conclude, if you want a 1960DM, save yourself a few quid and buy a used 1960 and a some used (and broken in) V30's from Ebay. The tone will be close enough that you'll probably never notice the difference, plus at the current prices you'll be £100-£150 better off than buying a used Mustaine cab. Unless you really have to have a cab with carbon fibre effect tolex and a metal grille that is.
There is potentially also wood density/rigidity and corner jointing to take into account, that can make a very pronounced difference on whether speakers might start smearing out with movement etc. knowing Marshall though, i doubt they go to much length to do anything particularly different.