Only refering to the original MS-50 here, no experience with the HX One (former Floor now Stomp owner, though, but still...) or the new MS-50.
- Quality. Some Zoom FX are nice, some are really bad, most notably all things dirt.
- Handling. Really great on the Zoom, but you have to select whether you want to toggle between presets or bypass a single patch. With the Boss and an additional switch (no option on the Zoom unless you cobble together some MIDI-to-USB stuff) you can have both, which I think is excellent. Just that it's limited to 2 FX you can switch between. Which is sad news.
My main point would be quality. At least the original MS-50 is outdated in many areas.