Functionality usually comes first and aesthetics, second. First off, I want a toy that works; a pretty toy that doesn't do its job isn't worth it. That said, there are usually enough (roughly) functionally similar toys available that it's possible to find one that works well and looks good.
Note, however, that in some toys, functionality and aesthetics are effectively intertwined. If I'm getting a BDSM toy that's primary purpose is to intimidate, the way it looks is crucial to its effectiveness.