I have gotten pretty close to MOA groupings with my 858 and Norc ammo, czech crate ammo is usually between 3"-4". These are not cold barrel groupings either. Most people will tell you that the Mini-30 does an average of 5" no matter how good the ammo (I say most cause I'm sure some people get better groups).
If you remove the extra spring in the 858 the trigger is VERY NICE for a military type trigger. Even with the extra spring the trigger is nice but heavier. Don't know about the Mini 30 triggers.
The VZ58s that I have shot have all eaten whatever they are fed without issue. I'm sure the Mini-30 will do the same.
Mini-30 mags may be hard to get. VZ58 mags are fairly easy as most rifles come with 4.
Far as I know the mini-30 is more expensive then the 858. That said the 858 is a superior rifle in many ways. I would always choose the 858.
YMMV