Originally posted by FortyJ
Strider, I believe the point is moot, because we are likely to obtain both tech advances from the Great Library. But let's compare the two:
Harbors
Harbors 1) provide extra food from the water tiles; 2) provide the ability to trade overseas; and 3) produce veteran naval units.
It certainly appears that our world is a pangea. If this proves true, then I would think that this would severely reduce the tactical need for veteran naval units.
During peacetime, the trade benefit of harbors will only apply to those civs that also have a harbor but do not have road access to the worldwide trade network. During wartime, it will provide trade to those nations with harbors that are separated from us by an enemy nation (provided there is still a safe route by sea). Since we are appear to be on a pangea, building additional harbors does not offer any additional trade benefits (unless perhaps they are on opposite coasts).
Which of our coastal cities are in need of extra food production from the sea in order to grow? If I'm not mistaken, only Morgana could noticably benefit from such an improvement.
Marketplaces
Marketplaces 1) increase tax revenues by 50%; and 2) increase happiness generated by the third luxury and up.
They can be built in all cities, not just coastal cities, and in most cases, the extra revenue generated from the marketplace more than pays for its maintenance costs.
We currently have one luxury in our trade network. Once we make peace with Azteca and gain the city Tlacopan, we will add Furs to that network. And, as soon as we build a city in the incense fields SE of Tenochtitlan, we will add a third. Each city with a marketplace would have four "happy faces" instead of three, which means one less garrisoned troop in that city is needed to keep the peace.
While I agree that building a harbor would be wise, I firmly believe that marketplaces are much more beneficial to us at this time.