One thing I enjoyed very much about the Soulless series is how romance is handled in the steampunk world with wild and crazy inventions and schemes alongside Victorian manners. That's missing in the YA series because it's for younger readers and, in the first book at least, there isn't much in the way of romance. So that felt like a missing piece, although it may be developed in later books in the series. In both series I like the way Carriger balances the mannered society against the need for werewolves to be naked and for vampires to drink blood. And the fact that fashion sense seems to top everything else in terms of priorities.
Overall this is a highly enjoyable read, particularly for lovers of steampunk and those who are new to Carriger's work. If you like the genre and don't like YA, though, perhaps turn to Soulless first.