We've got a couple of Cooper S's in the family and we love them.
However, my son who was 22 when he bought his first Cooper S did find it tricky (and expensive) to insure his S because in Australia car insurance on any forced induction vehicle for an under 25 driver is very very expensive (also difficult for a younger person to afford if on a tight budget). I would imagine the same price increase for a young person driving a performance vehicle would apply regardless of which country you live in.
You didn't mention the age of your daughter but it might be worth a phone call to the insurance company before you kick any tyres and check to see what cost penalty she might incur if you go with the "S".
But if money is no object on the other hand, then she will love the extra zip that the S offers. Once you've driven an "S" you'll never go back to a standard Cooper!! :-)
That's my 2 bobs worth but I wish her good luck with the purchase no matter which way she goes.