Is there anyone who's familiar with both and would like to share their thoughts on the two?
Arguably the biggest difference is that XTDB has a schemaless, dynamic core. Having a sane schema is of course important when building complex things, but we believe that the flexibility to experiment with schema (fully) in userspace is essential for moving the state of the art forwards. For instance a lots Clojure users these days prefer relying on https://github.com/metosin/malli for _all_ their schema needs.
Datomic's data model is something they call the 'universal schema' where you just specify the attributes and then create entities however you like from them. Datomic aligns more with how you structure data in Clojure, avoiding the impedance mismatch.