I will say this much: I had a very similar trip, though more aimed towards Universal. I would always, always, always suggest two or more days at Universal. I am an avid, rabid fan of Disneyland in California, and have been to Walt Disney World on nearly a dozen occasions. Still, I find Universal to be a far superior resort to the Florida Disney one. Disney World is fantastic and immersive and spectacular and grand, but I suppose I'm old fashioned: I like Disneyland and Universal Orlando for the comfort, closeness, and interrelatedness of the resorts. They're small and delightful and unique. And I can say (as I probably said in the other thread) that Universal's Islands of Adventure goes above and beyond anything you'll see at a stateside Disney park as far as themeing, immersion, and thrill. It is, without a moment of hesitation, the best theme park I've ever been to (and that was before Harry Potter came along).
Universal Orlando Resort uses the close proximity of the parks probably better than any other resort I've visited. You enter into Citywalk (which is free and open to public access). CityWalk is horseshoe shaped. Universal Studios Florida is at one end. Universal's Islands of Adventure is at the other. I like the setting that that creates, because you immediately choose whether to head towards the Universal globe and studio arch (Universal Studios) or the crashed seaplane and lighthouse (Islands of Adventure). It's a cool notion. As far as CityWalk, it's much like Downtown Disney or any other entertainment district: shops, karaoke bars, "clubs," theaters, and restaurants. Others can probably give you better advice on which are the most fun, but it's not something you'd plan a day around. If anything, you'd probably just stroll into a restaurant after you leave the park, go to the Burger King Whopper Bar for lunch while you pass between the two.
The parks are not busy in January or February. It's one of the slowest times to go. The thing that caught me off guard is that both Universal Studios parks closed at 6:00. It seems odd, but by 5:00, every single ride was a walk-on, so I could understand why. Just check the parks' operating hours and factor that into trip planning. A 10:00 to 6:00 day is 8 hours. That's much different from a 9:00 to midnight day, in which you have 15 hours in the parks.
As for the Disney Parks, it's a tough choice. I'll say that, on my trip, we used a one-day park hopper to get as much done as we can and chose to visit all of the parks except The Magic Kingdom. That's because we were returning visitors. If you have little exposure to Disney World or have never been, The Magic Kingdom is basically essential. My favorite of the parks is Disney's Hollywood Studios, and it has its fair share of un-missables (like Tower of Terror, Rock n Roller Coaster, Voyage of the Little Mermaid, The Great Movie Ride, Muppet Vision 3D, and Star Tours). Animal Kingdom is also a really incredible park, and Epcot has a few really awesome attractions too. I can say that if I could construct a perfect limited-time visit to Disney World for myself, it would be three days: a full day at the Magic Kingdom, another at the Studios, then a day split between Epcot and Animal Kingdom (despite those two being the largest parks, they can be tackled from a "thrill seeker" point of view fairly well in five or six hours apiece).
Also remember to factor in transportation. At Disney World, you should plan at least 30 minutes to get between parks because of the transportation system. To get from the Magic Kingdom to Epcot, for example, you must take a monorail to a connecting monorail. From Hollywood Studios to Epcot, a boat is usually easiest. To Animal Kingdom, you must take a bus. And if you're staying at an off-property, non-Disney hotel and not renting a car, figure out the hotel's shuttle schedule. For example, my hotel's shuttle went to Disney one time at 8:00 AM and returned one time at 9:00 PM, with the pick-up/drop-off spot being at Epcot. That meant that, by 9:00, we had to be back at Epcot, and that's something you can't miss. Even if you're driving, consider a hotel shuttle to save $14.00 on parking if it works in your schedule. If your hotel is near Universal Orlando, it's a pleasant walk along the waterfront to the parks, or a brisk trip through the massive parking deck with moving sidewalks - another way to save $14.00 a day. The areas around the resort are built for pedestrian traffic from nearby hotels.
The dolphin swim package is great at Discovery Cove, but if time (and not money) is your only issue, know that you'd honestly be hard-pressed to spend more than a few days at SeaWorld and stay engaged. It's a beautiful park with much to offer, but even at a leisurely pace I think you'd be just about finished at the close of day two. Everyone's different, but I understand that you're looking for a more thrilling, fast-paced trip with a teenage age range, so in that case, SeaWorld is certainly a full-day attraction, but not something you'd want to visit for three, four, or five days. Again, that's my opinion.
Wow. I just typed a lot. Sorry!