I could see Gumby being preferred with a colder/clinical sounding amplifier as it is a touch warmer than Yggy. I can't see anyone preferring Bimby to either... Mimby I could definitely see, as it's a lot cheaper, and arguably doesn't take as long for thermal stability/warmup.
Mimby is also a decent trade off if you're just driving a headphone amp... Coming from Mike Moffat himself, Bimby/Gumby/Yggy have more sophisticated sounding bass, however it may not be apparent until you're running a speaker setup.
I don't prefer Gumby to Yggy myself, as Yggy is ultimately more revealing/precise without sacrificing musicality with the amps I've used. The price was the deal breaker for myself, however if I were to recommend based on price alone, Mimby is clearly in a league of its own.
To put it more bluntly, I have never forgotten the sound of Yggy the three or four sessions I listened, even after owning/using Gumby since it was released. Yggy is clearly superior to discerning ears.