I use Alamo and Enterprise, but they share a fleet, and so far, I've had these cars in the past year:

Premium Car (PXAR):

2015 Chrysler 300 Limited RWD (V6)- Alamo San Antonio International Airport (SAT), 07/2015, 6,500 miles on the odometer, TX plates.

They also told me I could pick from any of the following cars in the Premium Car (PXAR) class:

2014 Chrysler 300 (Unknown Model)
2015 Buick LaCrosse w/ Leather (V6)
2015 Chevrolet Impala LTZ (V6, I think)
2015 Ford Flex Titanium (V6)

Premium SUV (Unsure of the class code):

2015 GMC Terrain SLT-2 FWD (V6)- Enterprise Orlando International Airport (MCO), 07/2015, 9,500 miles on the odometer, FL plates.
2016 GMC Terrain SLT AWD (V6)- My local Enterprise location, 11/2015, brand new off the truck with 7 miles on the odometer, MD plates.