Maybe you already have all the info you need on this, but I love Settlers, so I thought I'd throw my opinion on here.
Cities and Knights is EPIC. My family loves it. Commodities add a layer of strategy you just don't have in regular Settlers.
Seafarers, in our opinion, has a few flaws that are huge drawbacks. One, it's really hard to make random boards quickly, because you can't follow a pattern for the numbers. Two, the game goes really really slow resource-wise, since you spend time building settlements on coasts (touching fewer numbers/resources) and you spend time building lots of ships to get to islands.
I've never played Traders and Barbarians.