Ive had both for quite a while now… XMX during dev, and bought MM via pre-order.
note: Ive created vcv plugins in the past, hence a reason im interesting in MM.
tl;dr; they are quite different… both with pros/cons.
I was planning on doing a video comparison… but Im not sure, as frankly, I found a bit too much bias when discussing on other forums.
to the point where a discussion seems pretty pointless/uninformed.
first… 4ms did a fantastic job on MM, Ive nothing but technical respect for what they achieved.
MM main value proposition is number of modules due to ‘easy’ porting from vcv, and a certain familiarity of workflow. I also like its 8 cv/audio output (over xmx)
however, for me, vcv is also its achilles heel… in both workflow and performance, and the hardware is ‘ok’ but lacking in quite a few ways.
overall, Id say, its what I expected, but has disappointed me somewhat… but for reasons I did know before I bought.
I personally think the XMX is a better hardware platform (though I wish it had more ram) .
buildroot linux is a much better design, coupled with its much more powerful cpu.
things like usb audio interface are really useful, encoders are better than pots in this domain,
ofc, you trade this for less modules… though their UIs are designed for the small screen unlike vcv modules.
I do think the MM will appeal to more users… partly because its easier to envisage what it tries to be.
but underneath… its kind of odd…
its kind of cute having the vcv panels on the MM, but actually they are often too small to actually be of much use. most of the UI has you selecting parameters and jacks from a text list.
so what functional purpose does it have? esp. on a display which (like XMX) is small enough that UI space is scarce/valuable.
but ‘pretty’ sells … and I do kind of like it 
(note: if the MM had a panel the size of the SSP, with similar buttons / controls… it’d be a very different story!)
anyway, Im excited by both still…
the current MM ‘sdk’ is too limited for ideas I have, but hope 4ms will improve.
the xmx, well , I need to release what I already have … also messing with vcv has inspired me with ideas for other modules.
(my framework means generally modules will be able to run on both ssp/xmx with only small UI additions)
… so could be a busy winter 