I'm sorry but there is no debate, go for the S, you will never regret it.
Have you had a test drive yet?? I think not or you would have already ordered it.
Having driven both and having just traded up for the S i'm more than happy.
If you want that bit of extra zip etc then don't delay.
I'm surprised you only got 13k for the cooper.
Anyway, that's my view, you will never look back.