I was lucky enough to live with both the ARC Ref 160s stereo and 160m monoblocks for over two years. I have auditioned a couple other pairs as well. The differences seem pretty universal when the only difference is stereo versus monoblocks.
The monoblocks have a deeper wider and more defined soundstage. So the images are more stable and smaller, set in a larger soundstage. They are quieter (lower noise floor) helping to allow for the better imaging and soundstage. The stereo version has a bit greater impact as the power seems to be more concentrated on larger images across a smaller, shallow soundstage.
A big advantage of monoblocks is that you can locate them right next to the speakers and get really short speaker cables with will increase the dynamics and transients over a long run of speaker cable.
I think most people audiophiles would always prefer the monoblocks. However, folks that buy MacIntosh tend to value impact and slam over fitness. So, depending on your likes, it could go either way. If you drive a Corvette, get the stereo version, if you drive a Porsche get the monoblocks.
Sorry, I just have to say it... if you fall into the later category and are into highly nuanced sound and a system with great finesse... is this the right brand for you?,