Atwork
Immortal
I would only recommend internal trade, much safer and gives you a huge production and growth.
Whether to trade internally or externally depends on the circumstances. In my current game as Assyria, I have huge production and a good amount of gold, and I have the capacity to grow quickly, but I can't seem to scrounge up much surplus happiness. For nearly the entire game, I've limiting my surplus food as much as possible.
So, in this game, it would not help my situation whatsoever to trade domestically to build a large surplus of food in any of my cities. But, that is subject to change. Once I finally get circus maximus built and some zoos built, then maybe a domestic trade route or two will make sense.
As far as overseas routes, I prioritize trading with cities that my navy can keep an eye on. Luckily, I have a few great port cities that I can trade with, which are within reach of my scouting ships. And I have a small fleet that I can send in support, if need be.