Maybe because I am something of a newbie, I can try and break it down and get rid of some of the confusion. Right now you have more than a few choices:
1)Conventional. With conventional, you use a transformer to vary voltage to the track via a handle or some such, has a button to allow reversing the train, and another usually spring loaded switch or button for the whistle. Every type of engine, outside the original lionchief (and it looks like Menards) support conventional operation. From a cost standpoint, prob the cheapest, just requires a transformer with the right power. If you want to run multiple engines, requires block wiring (isolating sections of track, and a selector switch between like A and B output on a transformer). You select in each block which transformer arm controls what.
2)Lionchief - uses constant voltage to the track, like any command control. Has its own remote that comes with the engine, has a remote that allows controlling up to (I think) 3 engines. Lionchief + allows operating via bluetooth from an app on a pad or phone. Lionchief 2.0 adds the ability to be controlled from legacy.
3)Legacy/TMCC TMCC was the original version of Lionels control system and is what is in the ERR upgrade package you can use to convert engines to command control,Atlas and Third rail offered it as well. Legacy is the next generation of this, has more features. A legacy command base can control any tmcc or legacy engine. The system itself broadcasts its signal through the house ground wiring, the only connection is to the ground for the layout.
There is also something called a power master that can vary voltage to the track, the way a transformer handle does. It is fed constant voltage, can be addressed via the legacy remote and it varies the output to the track, allowing speed control, reverse and whistle for a conventional engine. It has the same limitations as conventional, you would need block wiring for multiple engines, each transformer output would have its own powermaster attached.
It cannot control DCS equipped engines or Lionchief or lionchief +
4)DCS DCS is MTH command control system, their engines (PS 2.0/3.0, roughly analagous to TMCC/Legacy) are equipped to get the signal. With DCS, the command signal is sent through the middle rail.On a larger layout requires multiple power districts, each with only 1 dcs connection to it
DCS also can control a TMCC command base (I assume a legacy base as well), so it can control engines with tmcc or legacy installed. However, I don't believe it can operate all the functions, it is limited (someone else can fill in the details). So in theory you could install DCS, buy a legacy command base and hook it up to the DCS TIU (Track interface unit, thing that gets wired to the rails), and do it with one, some people do that. Atlas has started offering DCS equipped engines as well.
You can have dcs and legacy running together, they don't interfere with each other.
A lot of the answer is what engines you plan on buying, if you think you will buy lionel and MTH engines or Atlas or Third Rail products, you my want to run both. If you plan or have a lot of MTH engines and a few legacy or tmcc, then maybe go with the DCS running TMCC.
5)DCC is a standard command control that came out of the NMRA around the time TMCC came out or thereabouts. It is used in the 2 rail scale world in N, HO and O, and is an industry standard.Not an expert, but it means that you can buy a dcc control system from Digitrax and it will control any dcc equipped engine. I believe there are extensions that some companies build into their decoders and command systems that other decoders or command systems won't support, but there is a hard core standard they have to support to be compliant. (for example, if some company comes up with a decoder that supports the engineer making a rude gesture, might only work with their command control system) .
MTH in their later engines had DCC support (prob for those buying for the 2 rail market). However, DCC and DCS and Legacy are three totally different systems, so makes no sense to go DCC unless you plan on doing what some have, convert their engines over to DCC.
Hopefully this helps a bit. Me, I haven't quite gotten to this point, plus I only have one command control engine at the moment, but I plan on eventually running DCS and Legacy. When I wire I'll wire the layout as if I have DCS and Legacy, then hookup should be easy later on (yeah, I hear you GRJ, snickering with the others over there