I don't have any older Minis, just two of the 580-series. They seem good to me, but then, I have yet to scope either of them, and I can't crisply focus on the factory iron sight, so I cannot say what their accuracy potential might be. I cannot see MOA, with my aging eyes, so it doesn't matter if they shoot MOA, at this point.
Going by what has been said to me by friends with both a Mini and an AR, owned and fired over many years, the Mini is certainly a reliable weapon, needing little maintenance.
I own an AR15 HBAR myself, and used to own a Colt AR15A2 Govt Cabine. Well, the Mini and AR are, to me, ideally suited for different roles. Close-range utility, which DOES include defense of home and self, are well-suited to the Mini. Fighting wars, and long-range work, are best-suited to the AR15.
I live near salt water, and sometimes mess about in boats. Therefore, I like stainless Minis!
A Mini, with a standard rifle-style stock, fits into slim cases such as lacrosse racquet bags.
As for the subject of the original post, well, if both have serial numbers starting with "580-", you are good to go with either one. If only the Walmart Mini is a 580, it IS worth the higher price.