I think the general approach is the latter one you mentioned. I see apps behave that way on the market all the time - where there's some kind of free "appetizer"-version, and then a full version of the same program that costs something. It's almost never unlock/key based.

I think restricting the available levels to one or two is probably a lot easier for you to program as well - You can simply make a build where you remove check marks from those Scenes in the Build Settings. If you make the solution unlock/key based, then you have to add conditionals in the code that checks for the key all the time to make sure the user can't access restricted material.

As a finishing comment, since the market distributes apps only and not files of data, you'd have to make the key an app, i.e. a key-generator that people would then pay to download. And once that's done, what stops one user from sharing the generated key, or running the generator again to generate a different key? It would have to involve some sort of webservice-based security that makes sure generated keys are unique, and tracks a key's status as either used or unused. Nah, screw that. Go with the 2-level trial. ;)