Actually there's been no official announcement as to that, but in a few of the videos it shows Samus using a superpowered beam and transforming into Zero-suit Samus. So it's possible you'll be able to play as both. I''m really, really hoping they are two different characters, as I main Samus pretty seriously and would hate to let her go for Brawl. But I'm still psyched about Zero-suit Samus.