Kind of a tough call for me. The Black Pearl quest was longer and had more substance to it, but was also boring and tedious, and I can't say the reward (Leadership) was worth the trouble. At the same time, the El Patron quest was more fun, but wasn't nearly as long as it should have been. I did like the variety of tasks you had to do though, kept it from getting stale. The rewards were pretty good too(cursed blade + cursed blade loot drops).
I like both tbh. Black Pearl is longer, but is also very boring, while El Patron is more fun, but doesn't last long.
Just my thoughts.