I've used both an MM4 and the Nova Modulator.
I used a MM4 for many years, mine was a really early one which had the volume drop issue. I had it modded by Keeley which fixed that and made it clearer and more hi-fi sounding, it's a matter of personal taste but I much preferred it after that. I've heard that recent MM4s might have addresed the volume drop issue - not sure if that's a fact. If you don't have space constraints and like the fact that you can program 4 settings, the MM4 is a good choice. In particular, I liked the chorus, trem, phaser and leslie settings which were really good; the Uni-vibe was pretty ok too. the MM4 is also really easy to program - set and forget. also, if you don't need to combine effects, it's a good choice. That same MM4 I had is now being used by Momat from Astreal, last I heard it sounded great on stage when he was using it.
The Nova has some pros and cons. It's a more hi-fi sounding effect but I don't think anyone would ever be seriously disappointed with the sounds. I've got a modded CE-2 and a Retro Sonic phaser on the same board for convenience, and the Nova's chorus and phaser easily hold their own against its analog counterparts. The Nova has a great sounding trem in particular, and if you're so inclined, you can combine effects to have your own customised takes on a uni-vibe or leslie (although I wish the Nova did have those!). Basically all the effects in the Nova sound great - although you'd probably end up tweaking it more, as with anything with more parameters.
My only gripe with the Nova is, if you're intending to cycle through a few settings within a song, prepare to work on your tap dancing. Other than that it's highly customisable and has heaps of good sounds in it.
While vintage or analog modulation pedals have a certain undeniable mojo, if you're looking for a one stop all-in-one modulation pedal to save on pedalboard real estate, all 3 pedals are great contenders. No sense beating yourself up obsessing over whether it sounds exactly like whatever vintage or analog pedal - it doesn't, and probably no digital pedal ever will. get over it and past it, and I think you'll be happy with the end result. Frankly in a live setting or even on a recording, I think hardly anyone in the audience would be able to tell anyway, and even so - who cares! The gear doesn't make the player anyway...