[octatonic]

I've had both- still own the Modfactor.

In short, neither.

The Eventide is much, much better sounding than the MM4 but there are several operational problems with it that I have tried to live with but ultimately decided it isn't worth it.

The main issue with the Modfactor is one of level.
It is impossible to not get a volume jump with presets.
Eventide are aware of the problem and issued an update for the Rotary, which was worst affected, giving you level control over it.
The rest of the presets are unable to control in this manner.
It is infuriating considering the cost.

No update is forthcoming for this issue.