I'd say depends on the deal (I can't remember the terms of the Smashwords author contract). Let's face it, you're in the game to get published, right? And Smashwords are serious about e-books.
How much freedom to distribute elsewhere do you get? If I remember, the Premium deal makes sure you're on Amazon, B&N and a bunch of others, even if not prominently.
I have my own isbn's so I'm not in quite the same position (have you thought about applying for your own?), but if you're not published by someone, you'll not get sold, and famous.
