This is hard because both parks are excellent in their own way. I've been to hershey the most and it's certainly a favorite park of mine but I have to say Busch gardens europe has alot more to offer on all accounts. Great rides, great shows and a great atmosphere. I've only been to BGE once, just two months ago and I have to say I wish I could go there as often as I've been to hershey. This is not saying hershey isn't as great but there are alot of little things that BG does that hershey does not and that's why it gets the heads up for me and got my vote.
It was really hard to choose though, if you can I would say make it to both, they are both worth a visit or two.