Unless you have a compelling reason for buying a Mini-14, I'd get a good AR first, along with the spare parts, mags, carrying gear and so forth to use it almost indefinitely.
That said, if you live in a State that bans ARs, then the newer Mini--the ones with thicker barrels and upgraded tooling/manufacturing machinery-- begins to be a practical alternative.
If you simply want one, then I'd only consider the newer versions, unless you can buy an older one for cheap that is still supported by Ruger in case you need to have it re-barreled.
I wonder if Ruger would re-barrel an older Mini with the newer, thicker, Ruger barrel? That might be an interesting alternative, especially for folks with older Minis with ruined barrels, or those who can snag an older Mini very cheaply.
In any event, shoot it with the proper ammo, and see how it does before making any alterations. Bite the bullet, and send it back to Ruger for a lifetime's supply of factory-fitted parts, such as firing pin, extractor, ejector, and all the other associated bits and pieces. Turned out to be a great move on my part, because I also complained to Ruger about lack of accuracy (mine wasn't too bad, but thought it was worth a shot) and Ruger replaced the bolt with a carefully headspaced unit on their dime, and fitted all the spare parts to the new bolt. The Mini shot very well after that..