If the limit to 64 steps was not an issue, which would be better for my needs? The Pyramid or the MnM? My intention is sequence external gear with live polyphonic recording on multiple tracks that I can manipulate and mutate live.

I’d go for the MnM over the Pyramid, but I’m unsure if it can do polyphony in terms of you playing a chord on your keyboard and it recording it into the step=sequencer.
I would think it’s monophonic like the sound engine (well, until you go into 6-note polyphony mode).
I know step-wise it can do chords via programming.

In short, I’d grab another MnM in a heartbeat if I had the money; it really is a beautiful instrument.
I’m not sure how geared it is towards your needs, but someone should come along soon enough to say.