I'd say that it depends on what you're going to use your Microbrute for. If you intend it only for bass duties, a bass amp would work, and for leads and solos that really cut through, a guitar amp would do.
But a keyboard amp with a beefy bottom end is the more versatile choice, especially if you pair the synth up with a good multi-effect (or a few guitar pedals). You'll be able to get the clean Microbrute sound in all ranges, which neither a bass amp nor a guitar amp will deliver since they're designed to color the sound in different ways. If you buy the right amp it will also give you just as much bass oomph as a bass amp.
When rehearsing and gigging with my band, I run my Microbrute through a Zoom MS-50g into our PA system. I usually only have some flanger and delay from the Zoom, but when I want to play a solo with a distorted, guitar amp like sound, the amp models in the Zoom deliver any amount of dist and any amp/cabinet flavor I need.
