The MTH HO K4 Pacific will run on DC, DCC, or DCS. According to the tests and reports, it has a very low top speed on 12 V DC, and really needs 16 V to work properly on DC. The tests said it did quite well on DCC and DCS.
DCS is a proprietary command and sound control system, with design and manufacturing rights owned by MTH. It has some features that TMCC - the Lionel system - does not, but is not totally compatible with either DCC or TMCC. MTH apparently claims a patent for using emf to regulate speed in scale 1 MPH increments, so that no DCC or TMCC manufacturer can use that particular feature. There are also some sound features that are unique to DCS, particularly in how they are controlled.
In 3 rail O, if you want to run both MTH Protosounds and Lionel and other makes of TMCC locos, you will need some of both DCS and TMCC equipment.
In the 2 rail scales, the DCC is the dominant form of digital command control. The signal and power specification going between the command station and the locomotives is an NMRA spec so that all DCC decoders will work with all makes of command station and control equipment. However, the control signalling between the throttles and command station is allowed to be proprietary, so that one manufacturer's throttles will not work with another's command station.
To date, you cannot buy DCS decoders separately. You must buy an MTH locomotive with DCS installed. And you have to buy DCS control equipment from MTH to take advantage of the DCS features.
my thoughts, your choices