AAY, First of all your going to America. Fuel cost/consumption is not an issue. I know its very difficult to understand that coming from Europe, it is just so cheap. I had a 5.0 litre V8 when I was there as it was old and cheap to purchase. It cost approx $25 to fill. Thats about 20 Euro! I have a 1.3 litre car in Ireland and its about $40 to fill. Do you see what I am getting at?
Now you say your going for your "commercial training". I take it this means the CPL in which case this will take Max 4 weeks. In this case its worth renting a car over buying. If your staying a few months then without a doubt buy a car. Its easy peasy. A day will see you get your Driving test ( A blind monkey could pass), Insurance (nothing like as expensive as at home regardless of engine size, and Tag (licence Plate). Its not uncommon to be able to get a cheap and cheerfull car on the road insured and all for $1200-1500 all in. The distances involved and complete and total lack of public transport mean having a vehicle of some description is not optional.
The EFT cars are cheap. You get what you pay for, simple as. They are older and most don't have Air Con, being a milky white Irish boy as we tend to be! You might find this a bit challenging.
If your under 25, renting cars from the rental companies can be expensive. You will have to pay $25 extra a DAY, with Dollar for example. Most of the big Companies like Hertz and Budget don't allow under 25s to rent at all. So you will be paying a fortune to rent this way if under 25.
Hope everything goes well, good luck!