Cars at YLW

Need to rent a car for 5 days in Kelowna in the winter. Going to drive to Revelstoke. Want to get an SUV with 4WD.

Using free nights so might as well get the best car I can get (Exec Elite). Best options are premium car (most $$), Midsize SUV (next most exp), and standard SUV (cheapest).

Not sure why midsize SUV is more expensive. Is there any way to guarantee SUV?

Thanks.