When it comes to rental cars, a regular, plain, sedan is by far the best value, and 90% of the time, they are really all you need.
A Standard or Full-Size rental will get you into a very nice car, that in a lot of ways will probably more comfortable than a small SUV. In fact, not only would going with the sedan not spoil the trip, if I was given the option straight up of taking a Fusion or a Santa Fe, I would almost certainly go with the Fusion myself.
While there is always a chance, I'd say the odds of you getting moved into an SUV for free are pretty small, and while I'm sure you'd have the opportunity to upgrade to an SUV, I'd be a bit surprised if the fee at the counter is any less than the fee you're seeing right now. That said, it never hurts to ask, so you always could reserve the car, and see what they will do at the counter when you go to pick up the car, and move to the SUV if they can give you a price more to your liking.