This is always a fun question to field because it varies so much based on what you want. In December, we will spend six days at Uni and 4 at Sea World/Discovery Cove. We have also done 5 days at Sea World hotel with a midweek overnight at RPR, for 2 days at Uni. Based on how long we can go, we tour the parks differently. Seeing "everything" is different for each family.
You will enjoy yourself at all of the parks for as many days as you can go. Sea World (which I love) can be a one day park, or a 3 day park. Why, how much do you want to keep moving on to the next thing? Take the penguin ride. The ride itself - pretty cheesy, good for kids, no thrills. Then, the end "reveal" is awesome. I could easily spend an hour just watching the penguins do their thing. Wild Arctic ride is just dreadful, but I love the animals after it. The aquariums at Manta and Journey to Atlantis are phenomenal, but you can walk right by them as well.
At Uni, HP has a few rides, you could get lucky and ride them all in about 60-90 minutes. For others, they can stay in HP world for days amazed at the detail. For us, why rush through Seussland? Its so witty. For others - an easy skip, just more pavement to walk to get to HP.
As this is your first trip, stay onsite, spend a day at each park, check out all that the hotels offer, and spend a bit of time at CityWalk. Then head over to Sea World for a day, catch some shows and ride some coasters. That will give you a taste and set you up for trip #2, which will come. Keep an open mind and just enjoy! You will love it.