I'm sure SC will do well, but the success of ED so far is most likely down to the fact that Frontier have been developing this game in secret for many years now - Brabam said at a recent conference that the majority of what we see now was there even before the kickstarter. Think of that what you will, but Frontier have a significant head start on SC at the moment, and that is why beta is going so well.
Planned expansion so far are:
Walking around ships including fixing your ship, firefighting, boarding other ships etc...
Walking around stations and other venues...
Planetary landings... this one will be big - not only just in star ports on planets but they are intending to be able to explore the surface, mine, hunt game etc...
Much of the first person stuff
is already developed using code and assets from "Outsider", however right now FD are working on getting the space bit right - indeed DB has been cited as saying there will not be an Elite 5 and that ED will be running for years through expansion.
This all however depends on its success. Which I am pretty convinced will be wild
