I have never used or even seen an EXO, some here in BC whose opinions I respect have and are very positive about them.
I have not used an SG, did speak with them a few years ago, very nice, helpful, but, I just do not care for gray packs.
A buddy, going Stone Sheep hunting has MR, SG, and several other packs, one bought from me. He is short, stocky, bit of a gut and finds, as I did for many years, the MR and original Dana packs to be the most comfortable.
Others, taller, slender, wiry, have found that the MR line is NOT as comfortable as some others, but, overall, I can honestly say, again, that the SO packs just amaze me in terms of light weight, QC, comfort and simplicity. The new "Brooks" with a Merlin Talon, seems as ideal for hunting in the mountains as anything I have ever seen and I am considering one, although my alpine hunting days are pretty limited now.
One of the major issues we have here, IMHO, is that there is now such rapid development and improvement in these packs that newer models from the top makers often are much better than the one you bought 3 years ago. Soooo, deciding that one is the "best" seems an exercise in futility, IMHO.