We stayed for four days at the Royal Pacific Resort this past October. There are fully accessible boats that run every 15 minutes or so from the resort to City Walk and then it is a fairly short walk from City Walk to either of the parks. The parks themselves are closer to each other than the WDW parks, so it is reasonably easy to walk back and forth between them.
There are also fully accessible buses that go from resort to resort and to Citywalk as well as bicycle rickshaws who provide rides for tips. Now, I didn't actually see any of the bicycle rickshaws, but people who have been there at other times assure me that they do exist.
We mostly walked back and forth from the hotel to the parks, as the path is a winding one along the waterways with lots of tropical plants.
As for the parks themselves...Universal Studios felt reasonably small and pretty compact to me while the Islands of Adventure seemed to be bigger and more spread out. One huge advantage to staying on site is that your room key works somewhat like a fastpass to almost all of the rides and shows at Universal. You just show them the key and then you are allowed into their fastpass equivalent line. This means that you really don't have to plan for anything other than the show times because you won't be spending a ton of time waiting in line. And the room key works from the day you check in until midnight on the day you check out.
The people here at the DIS over on the Universal boards can probably give you a lot more information than I have.
My spouse and I really enjoyed going to Universal. It wasn't WDW, but it was fun with a whole different kind of energy. I am in a wheelchair, so I am not a good person to tell you what the distances are, as my spouse is my "pusher" and he doesn't consider it a big deal to push me back and forth between Epcot and the Studios a couple of times a day.