Welcome to the forum John :)
you're right in saying it's confusing!
the S Line spec gets the sport suspension fitted as standard which is a 15mm lowered suspension over the normal suspension. However you are correct in saying you can delete this suspension and have the dynamic (normal) suspension at no extra charge. This suspension is softer. On the S Line you also have the option of the "s line" suspension which lowers the car by 25mm but is a £325 extra.
On the sport model, standard suspension is fitted. This is what Audi call dynamic but Sport suspension can be fitted free of charge which lowers the car by 15mm.
So basically there are 3 suspension options:
dynamic (normal) standard on sport model and a no cost option on s line model.
sport - 15mm lower over dynamic suspension (no cost option on sport or S Line models)
s Line - 25mm lower over dynamic suspension. £325 option on s line model.
One other thing to take into account is that S Line spec has 18" wheels which will make the ride harder whereas the sport model will have 17" wheels as standard.
Obviously on the S Line spec you'll get a plusher interior over the sport model.
Hope iv helped and not made it more complicated.
cheers
steve