Who is in charge of what?
If you have done any game development professionally, you will learn that the game that comes out won't be the exact match of what you are thinking. You have to let go the control sometimes. This is not just your game.
When this kind of problem arises, usually people would do AB testing. Create both, and see what works better. Peer feedback is the most useful thing in video game development. I am sorry to say that most likely your idea sucks, and so is his (and mine, and everyone else). We rarely get the first implementation correct. Peer feedback is important as it gives you honest reviews if game is fun or not. You'd most likely end up with something completely different than any of you two could imagine.