[Rolling up my sleeves].
Next month, my family will be arriving in Orlando for our second split Univeral/WDW trip, and we've also done trips to each by themselves. Here's my advice:
1 - Absolutely stay at one of the Universal premier hotels for unlimited express pass. Then crowd levels and touring plans will practically be a non-issue, not to mention you can go without transportation if you'd otherwise be renting a car. (All the premier hotels are a 5-10 minute walk from both parks and CityWalk, and boat service is also offered.) The premier hotels are all fantastic: for a 16-year-old boy, I'd recommend Hard Rock Hotel if he's much into music or just enjoys a bit of noise. Otherwise, Royal Pacific if you'd prefer a relaxed, modern Polynesian vibe. Portofino Bay is lovely but perhaps a little too sophisticated to appeal to teens.
2 - Don't buy multiday tickets for Universal. Buy 2-park (or 3, if Volcano Bay is a must for you) Seasonal Annual Passes, espcially if you think there's the slightest chance you'd want to come back to Universal within a year of your initial visit. They cost no more than 3-day park-to-park tickets, come with some extra little perks, are blacked out only for Christmas/New Year's week, Easter week, and the month of July, and entitle you to passholder discounts on your hotel room, 30% for the premier hotels. There's a whole thread on the DISboards dedicated to keeping an eye on when APH discounts come out: book the room you want ahead of time, and then modify it to apply the discount when they get released. Have some second and third choices in mind in case the discounts don't apply to the first room you chose, but apply to other premier rooms you'd be happy with. Be aware that APH discounts often get released in "waves," so the inventory of APH-eligible rooms can and does change, and checking back often is a good idea.
https://www.universalorlando.com/web/en/us/tickets-packages/annual-passes/hotel-offers If you feel like the Seasonal Annual Passes are too expensive and you really just want 2-day tickets, consider getting the Seasonal Annual Pass just for yourself -- that way you can still get 30% off on your hotel room, as the discount only requires one of the people in the room to be a passholder, PLUS seasonal passholders save 10% when purchasing multi-day park tickets at the gate (e.g., for the other 2 in your party). The amount you save on the room/tickets should eclipse the additional monies you spent for the pass.
https://www.universalorlando.com/web/en/us/tickets-packages/annual-passes/uoap-types#seasonal-pass
3 - At both WDW and Universal, allot about 1 day per park -- that is, one day for USF, one day for IOA, then 4 for WDW. This will allow you to do everything at both Universal parks, and the majority of what's available at the WDW parks. Add another day (or dedicate a half-day out of the WDW leg) if you'd like a mid-week pool day break or want to visit a water park.
4 - Structure the order of the two parts of your trip in whatever way appeals to you, keeping in mind that the WDW leg is going to require significantly more energy, pre-planning and waiting in line. The Universal leg will be your relaxed, more spontaneous time. One of the main differences between WDW and Universal is size: if Universal is David, WDW is Goliath. This helps WDW in that it has expansive, more immersive and impressive lands within the parks, but hurts it in the sheer amount of walking and use of multiple modes of transportation it requires just to get around. Universal's smaller footprint makes for much greater ease of touring -- you can hop from one park to the other in mere minutes -- but hurts it by forcing reliance on a much larger proportion of screen-based attractions to make up for the lack of physical space.
