Get the 181-series stainless. Ruger put a higher polish on these very early stainless Minis. I have a 181-series stainless made in December, 1979, just one month after production started. It's a cool rifle.
If the finish was good on the 181 series, it must have been really good! The finish on my 186 series (1996) was head and shoulders above my 580 series (2009). If you held both recievers side by side the 186 has much softer edges, the flats and rounds are completely smooth, and the bluing actually looks like bluing. The 580 has much sharper edges and the flat areas where the rollmark is was much rougher. Almost 'frosted'. Although they are both cast, the 580 screamed it. The color was also more black than blue. They both were 100% reliable but a blind man could tell Ruger spent more time on the 186. I'll have to look at a 181 some day.