We are taking a town car service this trip. It will be our first time doing something like this, but it included a grocery stop and is going to cost $119 without tip round trip. From the horror stories I have read about the mears shuttle service to Universal (waiting an hour at the airport and then another two hours riding the bus to drop other people off) we decided it was worth the roughly $40 more to get direct service to and from the airport with the free stop for food.
I remember when we were setting up this trip that you can arrange for them to do different hotels if needed. So, you could arrange for them to pick you up at the airport and then stop to get food and drop you off at Universal. Then you could have them come back and drive you to Disney and then once you are done at Disney drive back to the airport. I am sure this will cost a little more than the round trip price, but it might be cheaper than a taxi. They can also drop you off at Universal and pick you up at Disney as part of the round trip price and you could just pay a taxi to take you from Universal to Disney.
Also, Magical Express is great, but with your trip, I don't think I would take advantage of it. It would be much faster to go directly to Disney from Universal instead of going back to the airport only to have to wait for your bus to arrive at the airport. Also, and I might be wrong on this, but I thought your could only use Magical Express on the day your flight arrived.