The difference between SE and E is substantial.

Probably the best benefit is the SE desk. The phone is answered quickly, and the agents are really good at getting you what you need. I have not had experience with the Elite desk since SE came into being. I don't know how well it compares.

SE can upgrade from discount fares internationally (E can only upgrade from Y/M, and Y only to Europe). SE can confirm upgrade at reservation, E have to wait until 72 hours. Finally, SE have an advantage over EP in that we get 25% class of service bonus on upgraded travel.

SE can book reward seats out of the revenue inventory in both J and Y, E only have access to the revenue inventory in Y. Also, SE are not subject to reward blackouts.

SE reward sales include both J and Y rewards, moreso than E. P have reward sales in Y only.

SE get guaranteed reservations on oversold flights, up to 6 hours before departure on Y and S fares.

SE have access to MLL when not travelling (except LHR, YOW and transborder MLL's).

SE also get better deals on cell phone rentals, and with car rental partners.