10 - 15%? Come on man, Usman's been impressive but style matters a lot. As far as body of work they're really pretty similar, and tbh Douglas' recent competition has been stronger. I know the latest UFC contender is by definition the most brutal vicious killer assassin we've yet seen, but Moose - Rory - MVP - Koreshkov easily stacks up with Usman's last few and Moose is obviously the most dangerous guy either has fought. Of course Moose won but Douglas took not much damage against a very powerful middleweight who is a strong wrestler and much more dangerous ground and pounder than Usman (as well as being a longer and cleaner boxer), and his counter-wrestling looked very strong after the first round (the fight overall was really not very one sided at all, Moose's biggest advantage was in cage control).
I might very narrowly favour Usman by decision, but Douglas poses a way more novel challenge to Usman than vice versa. Douglas has seen a ton of strong wrestlers and power punchers, Usman hasn't seen anybody with the huge power at range and kicking game of Douglas, and his hooks are also very dangerous. Guys who are used to having a (usually sizeable) length and size advantage like Usman very often struggle when the tables are turned, the one guy recently who he couldn't easily bully with his size and wrestling in Colby gave him a hell of a close fight.