Just off the top of my head.
HBOGo has way more content than xfinity HBO on demand. The ridiculous amounts of menus you have to go through to get to what you want in HD on Xfinity HBO demand. The horrible On demand interface. On demand has no memory of where you are in a series. On demand is slow and if you are watching a series it might take 5 minutes to scroll through all the episides to get to the one you want, and after watching that episode you now have to do it again except scroll through now one more row to watch the next one. If you hit enter but Xfinity on demand but since On Demand is so slow you don't know it was still scrollinging through the episodes you load the wrong episode and have to go all the way back and start over.