I've read and thought about these two quite a bit.  (This article only examines numbers for Swissies.  These numbers do not apply to other breeds.)  I've done some number crunching. Looked at it from a scientific view as if OFA and PennHip were two competing theories. And like all other theories, they aren't perfect. They tend to make broad suggestions about changing the world. 

OFA has his advantages and disadvantages. Advantages being that you get a grade (excellent, good, fair, etc...), and dogs do not get rated unless x-rays are done after 2 years of age. This is not to say that hips can't fall apart after 2 years. Disadvantages are that it is much more subjective, ratings on the same dog can change. There is also that option of keeping your dog awake which affects accuracy of the x-ray. 

PennHip on the other hand is a statistic. It tells you "how loose" the hips are, which are correlated with hip problems. There is no chart on the correlation between PennHip and DJD for Swissies, but there is one for Rotties, which are similar in size/structure. If you look at the distraction indexes that Swissies have gotten (mean at .55 and looking at ranges on the health database), most have anywhere between 5% and 40% chance of developing DJD. Those odds are better than raw chance (50%). Surprise! 

However, I'm guessing from the available DI and percentile information, the distribution of Swissy distraction indexes is leptokurtic, peaks high and narrow. In short, it means that dogs who are slightly tighter than average will be in the high percentiles. Dogs who are slightly looser than average will be in the low percentiles. PennHip never implies that the issue is corrected nor do I known if there are enough Swissies that have done PennHip to fix the issue. 

But if you look at the mean PennHip score, it is right at 20% risk of developing DJD, which is the same statistic that OFA gets for hip dysplasia (19% of Swissies have abnormal hips). That should make you feel better in terms of number crunching. 

As far as correlations between PennHip and OFA reports, it seems pretty low. From my point of view, PennHip doesn't have much predictive power unless you have a dog in the top 10-20%.  Pennhip scores in the 0-80% tile don't mean much of anything.  

It boils down to doing OFA (that is subjective and does not provide any predictive value) or doing PennHip (provides you with some sort of an idea, but since correlations are pretty low, it is more unreliable than OFA's subjectiveness). Layman's terms - OFA tells you whether or not your dog has dysplasia, but not the likelihood of your dog developing it. PennHip tells you the chances your dog might develop hip problems, but not an absolute yes or no. And keep in mind there are dogs who have tight hips according to Pennhip and moderate hip dysplasia. 

I wouldn't say that one is better than the other. They are both very different in methods and interpretation. In a perfect world, since Pennhip uses an OFA view, you could also get an OFA rating at the same time. Then it would only help us understand how to evaluate breeding stock better. Then again, it is probably like asking Biologists and Sociologists to play nice.