Hi dixieland,
I have been in Florida since January and have visited the majority of the theme parks on offer, including Seaworld and Busch Gardens. I have never actually been to Discovery Cove but have heard nothing but good reviews about the place, even if it is very expensive!
Seaworld and Busch Gardens are two very different attractions, although both have lots of animals on show; the watery creatures at Seaworld or the African safari-like animals of Busch Gardens. If you are after rollercoasters and other thrilling rides, Busch Gardens is probably best. There are endless rides on offer, some of which are suitable for children or the whole family, but there are a number of good rollercoasters available. SheiKra with its two vertical drops is highly recommended. Seaworld has two rollercoasters; Manta (like riding on a giant stingray) and Kraken. It also has Journey to Atlantis, which is watercoaster - think of a rollercoaster combined with a log flume. If it were me deciding, I would go for Busch Gardens every time!
I think VHG has overestimated on the driving time from Orlando to Tampa, which is where you will find Busch Gardens. It took us a little over an hour, so I think 60 to 75 minutes is a realistic journey time.