KLM is flying ERJ-190 regional jet so is subject to similar cabin bag restrictions as to that ATR-72.
Dublin has pre-clearance so US formalities are done in Dublin and arrival is as a domestic passenger in the US, so no standing around after 7 hours on a plane for US immigration and customs.
Unless you opt for the cheapest fare type you are entitled to 20kg checked + carry on + free seat selection on transatlantic leg.
Shorter flying time via Dublin as well