I’ve used activerecord on microsoft platforms and rails before, and datamapper on sqlalchemy and didnt have time to dwell on comparing/contrasting them, so backfilling…
The gist of it is in terms of abstraction levels, activerecord is closer to the database and datamapper is closer to the domain.
http://culttt.com/2014/06/18/whats-difference-active-record-data-mapper/
I was pondering about the activerecord being so convenient for REST apis and was looking at this complaining about the rails/activerecord performance… http://developwithstyle.com/articles/2012/05/23/lessons-learnt-from-building-a-rest-based-api/