Alamo has the largest lot at SFB, therefore generally the largest selection.
There is a caviet here; Alamo is partnered with a couple of the airlines that fly into SFB for vacation/package deals so the demand for Alamo cars seems to be higher. For example, we have flown into SFB on Allegiantair and found that almost everyone on the flight was renting through Alamo (since they provide air/car/hotel packages with Allegiantair)
With Alamo, you book a size (ie: midsize, full size etc) and then after check-in you get to pick the vehicle that you like from whichever aisle (size) that you booked. If you are an Alamo Insider member (free to Join), you can do the online check-in before you leave home, then just take that paperwork out to the lot and pick your vehicle (bypassing the lines at the counter). Kiosk check-in is also available to bypass the lines if they are long.
We have rented from
Budget at SFB, not bad if they have the best rate. They will assign a vehicle to you, you don't get to pick. Make sure you sign up for there loyalty program and Fastbreak to get through the lines faster (free to join)
Also we have rented from Hertz a few times at SFB when the rates were good. Sign up for the loyalty program (Hertz Gold Rewards) to use the short line at the counter. They will assign you a vehicle also, if you sign up for the Alerts, you will receive an email or text telling you what your vehicle is and were it is parked.
We haven't used any of the other rental companies at SFB.
Have used National at other airports and there Emerald Aisle is a favorite with us, but there rates have been higher than most on our last few trips. Join there free loyalty program (Emerald Club) Basically book a midsize and pick anything from the Emerald aisle in the garage. We have seen everything from actual midsize vehicles (ie Corolla) all the way up to full size cars, SUVs, minivans, we even got an Infiniti G35 one time.
Very easy to pick up a vehicle if you are an Emerald Club member, print your paperwork, take it straight out to the garage (don't have to go to the counter), pick a vehicle from the class that you reserved, load up - keys are in vehicles, drive to the exit, give them your drivers licence, Credit Card and paperwork. They check you out and you are on your way.
National, Alamo and
Enterprise are basically all the same company so car selection is very similar
Hope some of this helps.