Well, I've already noticed that it's never worth buying a set of 2. You can get a set of 3 for a little more, or vice versa, you can even get a set of 3 for less (like the Deco M5). However, I don't think that the prices for a set of 2 would be lowered because of that. Therefore, I would always go for the 3-piece set, because you get more for almost the same or less and you can quickly expand at home if necessary because you already have a third device and if two are enough, you already have a spare part at home if something should break :).