We did almost exactly what you are doing and will do it again in March or June. We stay at Disney (POR) for 4 days or more and then move to a Universal resort, or vice-versa. You probably won't need the car at Universal once you get there. If it's at the end of the trip, you could return it, but that's more hassle and time than it's really worth. We really loved having a car at WDW. Previously, we had always stayed at Poly and took the monorail, boats, or buses. But our last trip to Poly the buses were always very slow and the monorail was being worked on. I think we were only able to take it once or twice. Driving to the parks is great. We took the bus to MK from POR last trip, but since we do RD, we are going to drive. If you get there early, then you probably don't need to take the TTC tram to get to your car.
It's so nice having the car at the park. If you don't want to hold something all day and don't want to deal with room delivery or picking it up on the way out, just take it to the car. Even if the traffic is bad getting out of the park at close, which we really didn't see, you have a nice cold/warm/dry place to sit while you are waiting. If you take the bus or monorail, you will be standing and most likely exposed to the elements. We also leave extra water and snacks in the car. It's nice to have that just in case. I like to drink tea in the morning, but I don't like to use disposable cups. I can bring my stainless steel mug in the car, finish my tea before we get to the park, and just leave the mug there. I usually bring in a refillable water bottle too, and I don't want to carry both around all day. We usually eat breakfast in the room. In a pinch, we can do the same in the car much easier than on a bus. It's also very nice to be able to drive to dinner at a different hotel, instead of having to pay for a cab, which isn't as cheap or easy at WDW as I think it should be.