Nah, CD's. I was Suzerain of 3 Merchant CS with 6 envoys in 2 later all 3, I was getting +25-27 gold in each CD, in addition to the trade route, the Harbor buildings are better but the CD just blew it out of the water no contest, I still built a few Harbors in cities that already had a CD or needed the port but CD was #1. The Merchant CS bonus is a little bit OP, it should be only +2 gold and affect both Harbors and CDs, it is weird how every district but a Harbor has a CS that boosts it. But also half the map was blocked off by land so I couldn't use my ships that much. And like Encampments, it's not like I'm going to be training Fleets/Armadas anywhere but 2-3 of my main cities so the naval production boost is fairly useless.