If they were the same size I would pick BJ Penn, because overall he's the better skilled fighter. But when you take into account the height advantage (5'9 compared to 6'0), reach advantage (which Nick uses really well), and endurance factor I have Nick edging over. BJ is know to lose a bit of heart if he isn't dominating his opponent, and Nick's the opposite. Both have similar skills sets (Boxing, weak wrestling, good BJJ), but you have to add in all the other intangebles. Not to mention that I believe Penn peaked 2-3 years ago, and Nick is now peaking.