Reading between the lines
Probably under the hood (MIDI, AUDIO and stuff) is the same (since they are both based on Ardour 4).
4 Extra buses can be (as one Harrison forum user said) either great or not enough.
What is (mainly) different is the whole concept of what the EQ's do. Mixbus3 is based on their digital console DSP engine whereas Mixbus 32C is based on the 32C analog console.
So I see it (32C) as a new console emulation option that adds 4 more buses (of which I was one of the early requesters that MB3 have more of). I only wish that MB3 will get back ported to have 12 buses too, but since I bit the bullet on 32C I can have the best of both worlds (as options that is).
The main "gotcha" is that if you took an unfinished MB3 project into MB32C it is (almost) like starting over since the EQs are different and can't transalte back and forth. So if I were to try and decide which I liked for a particular piece I would choose to import stems/track into both and approach like I was attempting a mix at two different "studios" or DAWs.
Once I think I have a handle on MB3 vs. 32C I'll probably know which I'd do a particular project on and not go for both simultaneously.