We usually split our trips between WDW and UO also. As my son has gotten older, the trips are heavier on the UO side then the WDW side. Definitely worth staying onsite, and we always stay at UO for the last part of our trip. The express pass spoils us at UO, and I fear would make WDW less favorable to DH and DS if had to wait in long Disney lines after fast express lines at UO.
Really wouldn't want to go to Orlando and not do both parks, and now we are spoiled and feel the need to stay onsite at both WDW and UO. It is not cheap, but we know it is worth the wait to save up to do the trip the way we enjoy it best- that is to say staying onsite at both parks.
Long story short, stay at a UO resort, if there is a MG enjoy it, and do the UO portion of the trip after the WDW portion.