Get the Sea World/Busch Gardens pass for $99 and do both as many times as you want. I agree that you can do Sea World in a day, but if you missed some rides or shows the ticket will allow you to come back unlimited for 14 days.
Sea World has a new roller coaster called the Manta Ray where you lay on your stomach and feel like you are flying. This is might be the best coaster I have ever ridden, and I have ridden a lot of coasters in my life. Sea World has anther coaster that is really good too and a combnation water coaster that is fun but not very thrilling. Here is a video of a ride on the Manta to give you an idea.
http://www.metacafe.com/watch/yt-fPvajSmu9rg/seaworld_manta_roller_coaster/
There are many roller coasters at Busch Gardens including the worlds tallest Dive Coaster called Sheikra. It drops you straight down and is a huge rush. Here is a video of Sheikra:
http://video.google.com/videoplay?docid=3272018762486823341#
Kumba is another great coaster along with Montu and 3 or 4 other different coasters. In addition Busch Gardens is a zoo with lots of animals in very nice enclosures. The monkeys can climb towers and walk on rope bridges, there is an open area where gazelles, rhinos, zebras etc roam free. There are shows and mild rides here too. This park is a full day and easily could fill a second day.
I like Universal islands of Adventure too, but busch Gardens has it beat if you like thrill rides. Having said that there is a new coaster that is opening at Universal Islands of Adventure that looks great. They were testing it when I was there this summer but as of July it wasn't open to ride. When the new coaster opens it looks like it will be one of the best coasters in Florida and the new coaster alone might make Unviversal Islands of Adventure a must see park.
When we were in Orlando this summer we added the brand new Aquatica water park unlimited admission to Sea World and Busch Gardens tickets for only $124.95 (plus tax) total for adults and for children giving unlimited admission to all 3 parks for 14 days. The water park was a fun relaxing day. Do the big slides or just ride in the lazy river or lay in a lounge chair on the sand beach cooling off in one of the 2 wave pools. They have a water slide where you slide through clear tubes through the middle of a tank holding Commerson dolphins. These are the most unusual dolphins I have ever seen. They are black and white like mini killer whales and these are the only ones I have ever seen. It was a fun day, and could be a fun several days if you wanted to swim and relax. If your resort doesn't have a great pool complex this could be your home away from home every day. It is worth the extra $25 to be able to go to this water park as many times as you want IMO.
Here are pictures of the dolphins and the dolphin plunge slide ride.
http://www.aquaticabyseaworld.com/Site/Rides.aspx?name=Dolphin+Plunge