When both are on the top of their game, pork ribs are better. But I've had beef ribs that are better than a lot of the crappy Missouri/North Carolina pork crap that people try to say is barbecue.
Pork. I agree that a beef rib, cooked perfectly, can be better than a pork rib. However, it is so rare to find, that I will go for pork everytime, which is always consistently good.