Paul,
As Crandell said, not all decoders are the same and (definitely) some are better than others. I have TCS decoders in some of my engines and they are excellent and fairly easy to install.
First up - PINS; there are different types of installation methods, completely "do it yourself" or "drop in" decoders. Drop in decoders have a "cable" with a connector on it, a "pin". You basically drop the decoder in the engine and "plug it in" with little more (if anything) to do in terms of an installation. the "DIY) decoder requires you to solder wires from the engine onto the Decoder Board. That is the difference very VERY basically. Others will give you a more detail definition.
I believe the "Function numbers" of a decoder determine how many "additional" things can be added to the particular board. All sound and non sound decoders come with factory set (default) functions that give you the basics for a particular engine. The additional numbers allow you to expand on what the engine comes with by default. For example, and I hope I am right with this, if you have an engine with a decoder that doesn't come "default" with ditch lights, you can use one of the additional functions of the board to install ditch lights into the engine. As said, I hope that is right, if not, someone will correct me and offer a more precise explanation. In short, the 4 functions are anything yu want them to be.
Not wishing to push you onto one particular decoder BUT, TCS decoders make "drop in" decoders for various makes of engines, Kato, Atlas, Bachmann etc etc; however, you need to add Sound to that decoder. In other words, you need to buy a speaker and wire that speaker to the Decoder Board. ESU (LokSound) is a sound decoder and ESU makes numerous engine specific decoders. I have never used them so cant tell you about installation.
In short, if you are not bothered by sound then I would recommend looking at TCS and seeing what decoders they have for the engines you own. If they do have them, I would go that route.
As for cost, TCS isn't much more expensive than the others such as Tsunami. The extra few dollars they may cost are very well worth the money though and we are not talking about ridiculous, over the top additional costs either.