I think it comes down to the philosophical differences in application between the two. As others have mentioned the Exo K4 has multiple frame sizes compared to the SG. Additionally the SG is a minimalist philosophy. The bags have less organization and minimal zippers (IMO) and are designed for the lightweight idea. I haven't been drawn to the bag design for this reason. It doesn't offer the functionality I need. I personally don't enjoy a pack dump to find something I want because I can't access it quickly. Maybe it's just the way I pack? For this reason I went EXO. I never actually got my hand on the SG because even if I liked the belt/frame, the bags were a no. Same thing for Kifaru. I tried the tactical and a hoodlum, ended up selling it. I had both the EXO K4 and the kifaru side by side and trained with them. I didn't mind the tactical frame, but the bag layout is just too great on the EXO.
I really like my Exo K4. Carried 65 lbs of elk meat and gear a few weeks ago better than my Kifaru pack. Granted, my Kifaru was an older internal frame DT2. I hate a bag that doesn’t zip open and is only top loading. K4 just fits me so well. I went with the 23” frame since I’m not a tall guy. Still need to get some belt pouches and the gun carrier.
EXO K4 because you can find a frame size that fits you best and the packs are better when it comes to organization, both haul weight great. I hated the old K3 hip belt and liked the SG one better but with the K4 the crew fixed the sliding issue.