Gearslutz-member 24db already shared this:
“1. The Mk2 is much lighter than the Mk1, no metal in the upper case anymore, no rubber endcaps, everything is lightweight plastic. Good and bad, easier to move but feels flimsier than MKI.
2. The Mk2 can run off USB power solely, but for hardware MIDI to work (and for the Mk2 screens/lights to be 10-15% brighter) an AC adapter must be used ($25 at NI site, $7-10 on Amazon)”

The MK2 is deeper than the MKI, takes more table space.

There are more features on the MK2, such as the monitors. For some that's been sought after and is a welcome addition. Maybe for a minority of folks (including me) the monitors are a distracting clutter, so the MKI is preferable.

No choice with the S88. MKI only at this time. Too bad they're $1K each. 

Thinking about a MkII S25 if it has thirty or more keys and an audio interface 

Save a little longer and get the 49.

Nah, I've got the Mk1 49 and happy to. Metal casing, OLED-screens, and I can use its adapter if i want the Mk2 to brighten up its screen. Moreover, a Mk2 49 is larger and heavier. But since Maschine 3 has an audio interface, I do hope Mk2 25 will get one too, and some more keys while we're at it, 32 would be fine.

Some other idiosyncrasies. What are those five buttons on the left of the screen for if you don't have a Maschine? And that 4D-knob just doesn't seem to help either.