I've only played the MS-20mini once for about 15 minutes, but it sounded very similar to my vintage MS-20 , which is one of my all-time favorite synths.
Honestly, IMO,the MS-20mini wipes the floor with the Pittsburgh System 1. It can just do way more, and has more sonic character than the Pittsburgh stuff. (I've owned a few Pitts modules) Hell, the patching options would be more interesting on the MS-20.(sample and hold, reverse envelopes, ect...)
Plus, all of the actual 'synthesizer' functions are jammed into one module on the System 1. From what I've read, that module has some audio bleed issues, and I've read numerous threads with people having troubles with the MIDI2 module as well.(FYI:for all I know these issues have been ironed out, and this is all 2nd hand Internet info...)
Also, if you're leaning towards the eurorack format, and you end up getting into it, chances are, you'll probably end up selling the entire System 1, with the exception of the Outs module.
As far as the Hz/V issue goes, there is a euro module made by The Harvestman called the English Tear which will convert Hz/V to V/Oct.
If you had the System 1 and the MSmini, you could replace the MIDI2 module with the English Tear and have clear communication between the two, which is obviously the ideal situation here.
I'm a total eurorack enthusiast, and that's where I spend most of my time when making music, but my vote is for the MS-20mini in this situation.
BTW: last weekend I was recording my girlfriend, and we broke out with the old MS-20 to track a bass line. It hadn't been turned on in a couple weeks, and was dead-on in tune, right when we plugged it in.