MnM:

FM synthesis - love it, but i do have a TX81Z.
crispy/clean sound.
MIDI capability - a fantastic plus, but by no means necessary.
6 tracks is a wonderful thing.
i sort of prefer the aesthetic design to the A4.

A4:
reverb/delay - VERY important and it seems the A4 does it better.
some of the FloppyDiskPirates sound packs are incredible/inspiring.
more flexible sequencer, arps, & parameter locks.
easier to dive right into.
can expect more Elektron updates.
better suited for live performance ?