The service level of all three are pretty similar at TPA - just go for the lowest price.

Do NOT rent from one of the fly-by-night rental car companies located off-terminal, such as Payless, etc. The last time I made that mistake I found myself in a shouting match over their demand that I purchase $150 worth of insurance, even though I was paying with a Diners Club. I ended up heading back to the terminal and going with one of the majors for about the same price.