I'm planning to do a similar thing, and I'm considering creating a C#/.NET Windows application and then porting it to OS X using Mono. My application already has a completely (except for the title bar and corner buttons) custom-drawn user interface, so the cosmetic OS differences shouldn't affect me too much.
I'm not sure what you mean by leveraging the best of each platform in terms of frameworks and APIs and so forth. In general, writing a cross-platform application means writing to a least common denominator, and thus means not getting the best out of each platform.
