I don't know all of the differences, but capacity doesn't appear to be one of them per the water level selector and the copy below the Maytag logo on the left. The 612 has Maytag's version of an "extra large" tub.
With fewer buttons than the 712, that means fewer options, like you can't have a slow wash speed with a normal spin speed -- it's all slow, or it's all normal. Wash and rinse temperature selections are fewer than on the 712 too, with no warm rinse option on the 612.
I think the cycle selections on the timer dial are pretty much the same on both.
Others may have more detailed information about the differences in these two models, and about the dryer as well.
I think $150 is reasonable for the pair, considering that at $75, you're paying less than 10% of the price for a new and frustrating electronic junk front loader, and getting something that will easily outlast one -- even after already providing over 25 years of service.