Check one of my last posts where I indicate how many semi auto .308s I own.
I searched for a couple of years for an HK770 that was in very good shape and reasonably priced. I finally found one a few months ago and couldn't wait to get it to the range. After mounting a nice Vortex in an HK05 mount (which will run you another $4-500 for the mount), I was very disappointed in the pulse of the recoil and its accuracy. I shot about 8 different types of ammunition in it and still the results were not what I expected. It frankly was no fun to shoot.
So, I reluctantly sold the rifle (but kept the 05 mount) and spare magazines (which are also very pricey - especially the 10 rounders).
How that compares to a MAC I don't know because I've never had the urge to own one. I can tell you that a 770 in excellent shape without mount is valued at $1,500-$1,800 with the 3 round magazine.
no, the atf told mac that they were too easy to convert to full auto, and to be changed to fire from a closed bolt, but afaik the open bolts are legal to own(unless someone converted it) and have a premium price to them!
There is no doubt that the HK is the far superior in quality here BUT if it boils down to these two I would keep the mac. I've own both these two guns and can attest to the fact you can shoot the mac without hurting the resale value to much as opposed to the HK. In addition, the mac put a bigger smile on my face. Plus its an open bolt. Either way there is a market for both. Good luck with the decision