Identical. Ideally they'd both use the same dll or else your notion of including the editor in the game is also a good option. Personally I prefer external editors unless I plan to include the editor as a user feature, but that's just me. If you're using the same models in the game and the editor, though, just use the same graphics code. If your implementation is properly abstracted you should be able to pull the graphics code out with minimal fuss and build the editor around it.
How "close" to the game code should the editor code be?
Alternatively, if you have found a good editor somewhere that will work for your game then you'd save yourself time by just abandoning the WYSIWYG preference and using the existing editor. It's something you'd have to weigh against the limitations it would impose on you.