I'm debating between Sixt and Hertz. I have Platinum status with Sixt. If I book an IDMR - BMW 1 Series to pick up at MAD, what would I likely receive?

Also, I need to add a day to the end of the reservation, but it increases the price by 200+ Euros - the phone agent suggested informing the check in person that I'd like to keep the car an extra day. Is that ok?