I haven't been to either, but my first choice would be Hershey, just because I'd like to get another launch experience other than Steel Venom. It also looks like it's got a few good GCIs, and I've always like GCI. It all depends on your taste. Kennywood has the classic Thunderbolt, and Phantom's Revenge is supposed to be great.
This looks fun!
Originally posted by Freddie
I haven't been to SFA but from what I've seen at the SF/CF parks I have been to the rides do not run anywhere near as well as the rides at small parks I have been to. This is why I always say I think you need to give these places a chance even if they don't look appealing to you because I really think you're missing out on good rides (but maybe it is just my 'weird' taste in rides, I don't know).
Valleyfair's roller coasters run magnificently. It might be the exception though, I haven't been to too many CF parks, just VF and CP. But I generally agree. Parks not owned by major chain amusement companies may have a certain extra-care factor.