I have the System-1 for a few months and received my MS 20 Mini yesterday. Played with iMS-20 on my iPad for quite a while.
I personally would never trade one for the other, because I am finding both to be very different. The System-1 is a Roland, so it sounds well behaved, more exact/polished, still can be aggressive. It's not as "sweet" sounding as a Juno, for example, though. But with the Promars-plugout, you get easily into that territory.
The MS-20 on the other hand is a rude and dirty boy by nature. If I wanted a trance'y super saw, I'd grab the System-1. But if I want weird drones, squealy filter-sweeps or distorted electronic mayhem, I'd certainly got for the MS-20 and be there in seconds.
When I am noodling on the Sys1, it motivates me into nice melodies, chord progressions that immediately form into something along the lines of a "traditional" electronic song.
When I am noodling on the MS-20, it motivates me into weird electronic noises constant knob-turning to generate textures and sequences that I don't hear on the radio very often

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I love my MS-20 (Mini), but I noticed some things right from the beginning that might be worth mentioning:
- Only 5-pin MIDI input. MIDI In/Out via USB (not checked myself), though. Only notes, no controllers (not in and not out).
- It's noisy. Very noisy in my opinion. And the noise-source is before the VCA, so the noise goes up and down with your VCA-envelope, too. Very annoying. It disappears with higher VCO levels and the LPF open, but it's very audible otherwise.
- The VCO-2 pitch control is a single control over 2 octaves. For dialing in a slight detune between VCO-1 and 2 you have like 1mm of way on the dial. And for both VCOs to be in tune, it's not "0" on the dial but slightly to the left.
- CV is hz/V on the Korg and V/Oct. in Eurorack-Land. This hinders the extensions of the MS-20 with Eurorack-modules a bit. Can be solved by buying conversion modules for extra $, though.
To wrap up:
Two completely different synths that can do completely different things very well. So it depends highly on what kind of tones you want to get out of them.
I will certainly keep both.