Hard to say without more details but:

If your service has a real professional end user (i.e. someone who will make a living off your service), a multiple tier one seems a better fit. For example look at smugmug - they have their free account, a couple of paid accounts aimed at "serious" users, and a professional account aimed at people selling photography.

Evernote has only one paid service but that matches their usage base - either free/casual users, or people who have bought into the whole "second brain" idea. But note that the evernote paid account is still reasonably priced.