It all depends on what type of coasters you've already done. I just did both parks along with 15 others less than a month ago and I have to say Dorney Park was very poor in comparison to Hershey.
ALL of Dorney's coasters have trims, including Talon. The woodies were two of the worst on my entire trip (out of about 30 woodies on the trip). Steel Force is good (especially the turnaround), but is far from the best hyper, and the same for Talon.
Hershey is a far better park with a great atmosphere and two great woodies. Although Wildcat is not my favorite it has been rated as the world's no.1 woodie. Steel coasters are abit lacking, the Wild Mouse is better than Dorney's, Great Bear is about as good as Talon, SDL and Trailblazer are poor.
If you like woodies, and all round good parks, Hershey is the place.
If you want a water park and have never ridden a hyper then go to Dorney.
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Parks done 2001: BPB, PKI, SFWOA, CP, KW, HP, DP, Knoebels, Astroland, SFGAdv, SFNE, SFA, LC, Moreys Pier, PKD, PCW, BGW, DMP.