Well, we absolutely love staying on site at both locations, so this is what we are doing in December - Saturday to Friday at Universal (Royal Pacific onsite), then Friday to Tuesday at Fairfield Inn and Suites Sea World on a Discovery Cove Package. With admission to Discovery Cove, you get unlimited admission to Sea World and Aquatica.
If staying at Universal is an option, hop over to their boards and read through the resort page, lots of info!!!!
We really like Fairfield Inn and Suites because we don't need to rent a vehicle then. Some of the other partner hotels are a bit nicer, but we love the location. You get a nice continental breakfast, but withing walking distance (1/4 mile or closer) is BW3, Fridays, Panera, KFC, Wendy's, Taco Bell, pizza joints, chinese, hibachi, etc., so we have evening meals a plenty if not eating at the parks. The shuttle service is great, just have to stick to their schedule. Also, a grocery store and multiple drug stores nearby.
We are a Mears family, but Tony Hinds gets a lot of love as transportation providers. I use Mears extensively, and it always comes out to be within 10% of a rental, when gas and parking is accounted for. So, we choose the convenience.
Another really good option to consider is booking the entire time at a Sea World partner hotel (Discovery Cove package) for the perks, then putting the Do Not Disturb sign on the door for a night and taking an overnight bag to stay one night at Royal Pacific at Universal. Buy a two day ticket and you get Express Pass for both days at Universal. It is a great way to visit Universal!!