I'm torn. Rokushiki is incredibly versatile, with Geppou and Soru allowing for almost unrivaled mobility, and Rankyaku still being one of my favorite combat styles even as we approach Chapter 900. Haki, on the other hand, has Kenbunshoku, which I think is the most useful type of Haki. The ability to anticipate your opponent's next move, and to be completetly aware of what's going on around you is really, really useful.
I'd say it depends on where I'm at. In the New Wrold, I'm gonna have to go with Haki, just because EVERYONE has it over their, so without it, you don't stand much of a chance. It would be like picking a fight with the entire U.S. Navy when you don't even have a boat, only two floaties for your arms. The point I'm trying to make is that you can't win.
In Paradise, though, Rokushiki all the way. Even though Haki would still probably be the better choice, and I'd say Rayleigh would teach you more than you could learn from Lucci, Rokushiki would be more fair, and subsequently more fun. If Luffy had Rayleigh-level Haki in pre-Time Skip, it would just be overkill, and it would make the story less interesting at that.