Antioch made sense in MTW because of the way trade worked with the overland trade through Mesopotamia, especially the after the Marco Polo world event.

In MTW2, I suspect that Constantinople would make the best candidate simply because of the way the sea trade works. I have no concrete numbers to support that assumption, however.

In addition, as Kraxis noted, the more central location helps. Aside from the trade increase, your better merchants will have easier quicker access to the higher end resources, except for the silk around Baghdad of course. From Constantinople you'll be closer the amber around Vilnius, Riga and Novgorod which is highly lucrative. The faster you can replace deceased or acquired merchants, the better.