One merchant ship with five garrisoned traders has the same efficiency as two merchant ships, however 1 ship + 5 traders costs 500 food and 500 metal, while 2 ships only cost 200 metal. As all the units count towards your population limit, 2 ships aren't just significant cheaper, but they only incur a population of 2 instead of 6, you'd have with 1 ship + 5 traders.
Therefore garrison traders in merchant ships usually doesn't make sense at all and might be only helpful if you have land based trade at first and switch to merchant ships later on.