Finally, a question I'm an expert on.
We have done both Hershey and Bushe Gardens. As a matter of fact, on the off year's we do not go to Disney, we go to Busch Gardens in Williamsburg. Busch Gardens is a very, very nice trip. The place is beautiful and it is VERY family friendly. We also go to Water Country U.S.A. which is my 12-year-old DS's favorite water park (he likes it better than typhoon or blizzard). We love Busch so much that we brought season passes last August when they had a special (buy the passes in August and get the rest of the year plus the next year.) Because we live in NJ, it is not that far of a drive, last year we took our car on the ferry from Cape May, NJ which was fun for the kids and then drove up the coast, a very nice drive.
Hershey is nice but seems a bit crowded whenever we go. Busch is a lot less congested and a lot more along the Disney line. While Hershey is more like an amusement park, Busch Gardens/Water Country U.S.A is more like an adventure or theme park experience, like going to magic kingdom and typhoon lagoon.
Busch Gardens, Williamsburg has a European adventure theme. The park is broken into countries. France, Scotland, Italy, Germany, etc. They have some of the best rollercoasters. Don't forget to try my favorate, appollo's chariot. If you really want to make an extended stay of your trip, you can visit historical williamsburg. The nighttime ghost tours are great. Families go on a tour of homes and other buildings in town at night and the tour guide tells legends about each place. We stayed in Williamsburg for 8 nights and there was plenty to do the entire trip. I think your children will love Busch Gardens, mine did.
Because we have the passes from last year, even though Disney is our extended trip this year, we will be going to Busch Gardens in May and again in July when we return from visiting family in North Carolina. Maybe we will see you there!