Given that the Superman line only has 4 books, if you were to add another book, I think Steel is the best choice (don't see the need for another Superman book). I also think it would be a good idea since a lot of the books that have been and will be cancelled brought some racial diversity that is being lost. Question is, can he hold his own?
As much as I enjoy the back-up in Action Comics, that's not exactly what I'd want to see in an ongoing. It has some good monologue and establishing of who Steel is, but I think a full ongoing should be more high-concept driven and more ambitious. I think you can do a lot with Steel. Obviously he can be DC's Iron Man, but he's not nearly as hedonistic and has a more socially conscious slant. You can do a mix of superhero and cyberpunk, sort of Warren Ellis-esque.