I subscribe to both sites, and when my sub on ancestry ran out a few months ago, upped my membership to include the london parishes.
FindMyPast is invaluable for the 1911 census.
However, census searching is better via ancestry, unless you have a specific address.
When some census details (yes, the actual pages) were missing from ancestry, i found them on FindMyPast, and the resolution was much better.
However, I use ancestry for the london parishes, as the coverage is quite good, now.
Ships passengers out are on FindMyPast, but In are on ancestry.
But having found an ancestor on the Ellis Island site, and scrolled thru the actual pages on FindMyPast for ships out, there is no way to amend what must be a transcription error, as there is no way of finding out what they actually trancribed the entry as, in the first place.
military records are on FindMyPast.
regards