Well done with your test.
Others on here will advise better about any differences between the cars, I don't think there was any major differences, but I'm not overly familiar with the S.
It is quick though so with all due respect, being a new driver you would have to be very careful until you build up experience. The road holding is superb, but push it too far and you'll be in trouble, as with any car.
I would have thought the insurance cost might be a killer for you, until you have a couple of years more experience.
TBH, I've had a couple of coopers and have just picked up a cooper coupe auto, and it's plenty quick enough, sure, I'd like an S myself, but I got this one more a good price and love it, quick enough to be fun. Being a Mini, even a One is great fun.
Best advice, if you go for an S or something else, get some more tuition - IAM course will cost you about £150 via a local IAM group, great to do as you'll be able to get some decent insurance discount - do it now before you pick up any bad habits. One thing I promise, is that it'll teach you to get more out of the car, in safety.
Not sure where you are in the country, if your local to me (Berkshie) I'll take you out myself for a demo ...