BA silver or gold is useful on AA not only for selecting MCE seats for free, but also to have access to AA's lounges (very good for wifi, less for food) for which AA's fellow sapphires and emeralds have to pay to access.
Regarding MCE I can only say good things. I've only flown it once on a LHR-JFK daytime flight, the service is the same of economy, but at least you have more legroom and a slightly bigger seat. On board wifi and IFE were very good and made the flight enjoyable. On the return I was booked in business as it was a night flight so I can't tell how good (or bad) it is to sleep in a MCE seat. For sure it is not premium economy.