When you book you're told what vehicle it is, if you find you don't like the car, just cancel straight away and simply repeat the process.
If you're in the West end you'll invariably get some nice kit available so you can try a few times to get a car you like.
There is no cancel fee if you cancel within the first few minutes so you can have a few tries at it to try and get the car you desire.
I've found that its roughly 50/50 new/old S Class these days.