I’ve had both. The BS2 has a better UI generally but its keyboard was too short to be useful (for me) so ultimately it went. I don’t much like the Mono Circuit’s reliance on a single encoder for all modulation but it sounds decent enough. I’ve programmed a bunch of patches I quite like and can easily switch between them using buttons (I don’t use the sequencer much). At the same time I bought a JX-03 Roland Boutique which, despite its tiny knobs, I like much better. It’s faster to get around and I prefer the sounds I get from it.