IMHO, Revenge of the Mummy is worth the price of admission to US on its own . IOA is a park you must see, just dont ride Dueling Dragons 4-5 times in a row with no wait
Disney is very hard to do in 6 days, you might not be able to appreciate or take it all in.
My DS(12) much prefers US/IOA. If it were me, I would stay at Universal and make WDW the side trip. Staying on WDW property gets you use of their transportation, and EMH for one park each day. Staying in one of the 3 Universal resorts gives you use of their transportation, which isn't really that necessary because you can walk everywhere. It also gives you "express" entrance to all rides all day, and first available at restaurants in Citywalk. Not only does UO have cheaper season passes, but most of the time you can buy a 5 day pass for the price of 2 days. I would recommend Hard Rock Hotel for your kids ages. The pool area is great also. Check the Universal Studios board for helpful hints in getting discounts for the resorts. That said, I personally enjoy WDW more, but DH & DS think there's no comparison. They're both great!