Besides the obvious scale differences, what have you seen to be big differentiators between google and other ad exchanges?
adx is the cheapest, because it's the worse. your ad will be seen by users in spots previously occupied by "the banks hate this guy" kind of click bait ads. which have ridiculous ctr!
everyone I run experiments there I keep getting calls from clients to take their ads out of some and some sites before brand sees it. it's a nightmare.