We stayed off-site for Disney and stayed on-site for Universal, and absolutely had the best time!
The hotels are great at Universal, we stayed at the Potofino and paid 150 per night.(HRH could be got for 125, new hotel for 99, all entertainment rates)
The front of the line acess you get is well worth it if you can work it. I'm glad we did WDW first, even though the lines were really small in early May, we absolutlely did not wait for anything at Universal or Isalnds of Adventure. My kids, 5 & 10 loved these two parks even a little more than Disney. We rode lots of things at least 5 times, and MIB 15.
We spent three days at these two parks, and I'm not sure we coulod have had more fun.
Of course, it was all fun!!!
September should be a slower time too, if you can't stay on-site at Universal, and it took us 15 minutes to drive from our hotel near Disney to the Protofino hotel at Universal (we left at 8:00 am)
We rented a car anyway, 230.37 for the whole 10 days we there, National.