I am not an Expert, but from the highly detailed discussions amongst
experienced users of the Mini-14 in .300 BO, vice
experienced users of the Mini-30 in 7.62x39, the overwhelming majority of folks who have experimented with
both is that the 7.62x39 Mini-30 is
usually the way to go,
unless one wishes to attach a sound-suppressor to the platform, and then the (usually) subsonic .300 BO gets the nod--for that particular application. OTOH, one can handload 7.62x39 to subsonic levels. The discussion of
subsonic 7.62x39 vice
subsonic .300 BO is not well defined as yet.
The only caveat concerning the Mini-30 is that Ruger seems to make them with firing pins which are just a little too short for 100% reliable ignition of
some steel cased ammo. The solution is to go to
Firingpins.com and buy a slightly longer firing pin. It
may be necessary for user to
carefully "adjust" the firing pin protrusion. Search "mini-30 firing pin protrusion" on
Ruger Forum for details. Suggest that if shooting corrosive 7.62x39 ammo, the Stainless Steel Mini-30 is an advantage over plain steel.
The "Dirty Little Secret" about the Mini-30 is that they show up as used rifles in gun stores fairly often because their owners have become frustrated with them due to ammo ignition problems and get rid of them. Now you know the "rest of the story".