You do get extra baggage allowance, you do get priority checkin, priority security access, priority boarding, and you will have lounge access. The Copa Club in terminal 2 is gorgeous but the food is terrible. The Copa Club in terminal 1 has terrible food but it's a least a little bit quiet.

You should have free seat selection if you correctly input your UA MileagePlus number into the reservation. If you can't do it online give the Miami call center a call and they'll do it for you... unless you got basic economy. You don't get the extra legroom seats for free in advance. If they have them available at checkin and you ask very nicely and point out you are *G, sometimes the agent will do it for free, but don't count on it.