I recently decided to switch the arguments in Interpolations.jl from types to instances, although in that case it actually made the interface a little uglier rather than prettier. The main reason was that I suddenly had the need for the flexibility provided by allowing for instance fields on the flags. My takeaway is that even if it's slightly more to type for the user, it's probably worth it in the long run.
(there's also a note on this in the style guide section of the manual...) // T
