One major difference between the two, and a very important difference, is that one is subjective (OFA) and one is objective (PennHIP). Chances are you could have the same x-rays read by different OFA reviewers with different results. PennHip uses replicable measurement methodologies that eliminates the subjectivity. PennHIP is a predictor of the potential for bad hips. It is not a pass fail, it gives you ranges whereby the higer the score the more likely it is that the dog will develop bad hips. The predictive validity of the test has been validated using thousdands of dogs in academic studies that have followed them out several years. Unlike OFA you MUST send in the studies to get the results. Reviewing the films and deciding NOT to send them in for review is not an option, which further strengthens the validity of the results.
In my opinion OFA has become a relic. It has no predictive abilities, it simply tells you if your dog is dysplastic at the time the films were taken, but is subject to errors by both the techician and the person reading the films. The difference in cost is well worth what you get out of PennHIP.