Honestly, one of the selling points of the Mini is that it DOESN'T take AR mags. Mini mags may be more expensive, but you only have to buy them once. In the past they were scarce and VERY expensive making it a legitimate gripe against the Mini, but now that is no longer the case. They are somewhere between AK mags and AR mags in terms of durability.
My only knock against the Mini's mags is the more cumbersome change. Granted, I haven't practiced it much, but it's just never going to be as easy as an AR or even an AK. But yes, the Mini mags are much stronger. Not quite "use an empty to beat your enemy into submission" that you get with the AK, but they have a much more substantial feel. You don't get the idea that they're fragile, like you do with GI mags. Sure, Pmags, ARC mags, and the like have improved the STANAG situation, but I'd expect a Ruger Mini 14 magazine to be longer lived.
They are more expensive, but with STANAGs for the AR, you need to have several in case one goes down. With a Mini, you may only buy half as many, but that's still more than you'll need.