If I'm generating more merchants, I'll settle the first and use a latter one for a corporation. Otherwise, I might have to sit on an unused great person for a a few turns to make sure I can found a food corporation.
I rarely do trade missions--if you do the math, you generally get more money throughout the game if you settle, and the extra food is nothing to sneeze at. The only time you would need a trade mission is when you are in dire straights for cash. And this isn't "I want a new sweater that has an in-season style" dire straights, but a "Holy s&^#! I need to pay my electric bill so I can play Civ4" kind of dire straights.