This really depends on some different factors. Where are you staying at Universal? If onsite, then you may not need a car. Would you like to be able to go off property to have dinner or go shopping or something along those lines? Then you would need a car.
Basically, you have to figure out what you plan on doing while at Universal. I personally don't leave the property, so I use a taxi (about $30 one way) and then a taxi to the airport. If you rent a car, remember that you have to pay a daily parking fee at the on-site resorts, so that may factor into your decision.
If you are just visiting Universal and staying on Disney property, I would rent a car for those days. There are rental locations at the Dolphin, Disney Car Center, and a Dollar at the Holiday Inn by DTD. This would be cheaper and easier than taking a taxi/towncar/Mears.