I've read a few accounts that ECAS is more up-to-date than myCIC. That is, status changes tend to show up on ECAS a week or two prior to showing up on myCIC. But others experience the opposite.
ECAS claims to be updated daily, and I've read that myCIC is supposed to be updated in real time. It's notoriously tricky to link your account to myCIC, and the "help" guys are both very slow to respond, and not that helpful.
http://www.canadavisa.com/canada-immigration-discussion-board/-t471280.0.html