Know that if you travel from SOG, must pay for parking in those huge garages and then make your way to the parks. (though you pay for parking at the onsite hotel's too, but the lots are much smaller and the ride or walk from the hotel is nice)
But I agree, the 2 big benefits of staying onsite are 1. The express pass. It really is a beautiful thing to get on fast. 2. One hour early entry to Potterland. It definites does cut down on how many people rush that early opening.
In October, especially on the week day before school lets out will be not as crowded. There may be early closing though, because of the halloween scary nights, hard ticket events.
And, the 2 big rides that are not express pass are the Forbidden Journey in Potterland and Rip Red Rocket in Universal.
Either way, it's a nice diversion from Disney. The ride to get there is only around 1/2 hour, and I found the 2 Universal parks much easier to relax and go with the flow in. Not as many strollers and other wheeled vehicles. My kids loved all the rides. Most of the park is geared for older, so I think your kids will love the park too. We did everything non-Disney this year (Universal and the 4 Busch parks) and on the way home, the kids said can we do Disney and Universal next year and give up the other parks, but maybe do DC too again?