Tough call, depends on what you need it to do.
From a price consideration, MS is obviously cheaper.
MS can be used as a USB interface whereas Mortrix currently can’t (it’d require a yet-to-be-produced add-on module. (Innate USB MIDI capability was something I asked for in development but the hardware was kind of already specified too much to add it.)
Mortrix currently doesn’t have dynamic labelling; i.e. a tile doesn’t display a different message depending on what message is being sent. Again, this something I pushed for right from the outset but it hasn’t yet been implemented. There is at least now the capability of having a different text displayed depending on whether the preset is engaged or disengaged. There is also now a live edit control overlay for seeing PC numbers on Mortrix, but it’s not the same as the MS’s ability to have individual scroll counter readouts on any tile.
Mortrix has simpler button interaction than MS: Mortrix just has press and long press (latching or momentary), whereas MS can assign things to double-taps, long-double-taps. MS’s extra complexity means you can potentially do more on one button but it comes at the expense of potentially forgetting what button actions are in use and what they do. Mortrix is more straightforward in that button actions kind of always do the same thing.
MS handles expression messages on a per-bank basis. Mortrix handles them per preset, but they can also be re-used with a bank (scene) in numerous ways so this is much more powerful.
Mortrix is very intuitive to program, with lots of the tedium removed, and no need to connect to a computer to program it. It also has some amazing features that are not possible on MS, especially in the realm of expression, envelope, LFO and sequencer uses.