You can't lose - they are both great parks, traditional with rides you can't find anywhere else. Kennywood is bigger, with more rides, and has "the world's best french fries" (Potato Patch); Knoebel's is the best amusement park value in the world (FREE parking; FREE admission; bring your own food if you want!) with excellent -and reasonably priced - food. It also has a great dark ride (the Haunted Mansion), a beautiful setting, and maybe the most enjoyable non-coaster ride I've ever been on, their scooters (bumper cars). Kennywood has three great wooden coasters, and a huge steel one that, after riding once, I will never go near again. Kennywood has a really cool junior coaster, an excellent new woodie in Mr. Twister, and, of course, the Phoenix, which I feel is just perhaps the best coaster I've ever been on. So it's a tough call. As for me, having been to both, the one I'd return to (if I had to pick just one) would be Knoebel's.
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Age and treachery will always overcome youth and skill.