There are a few main differences between DCS and Legacy (rule out TMCC since it doesn't come close to either of the other two).
DCS has 2-way communication between every DCS engine ever manufactured and the DCS Remote. This allows the engines to feed back things like time powered up, scale miles traveled, the length of any given stretch of track, the engine's battery condition (PS2 only since PS3 has no battery), and track voltage.
Legacy is 1-way communication from the Cab-2 to the engine. Newer Legacy engines may have an IR sensor underneath the engine that can communicate with a Lionel Sensor Track, however, this is not standard for all Legacy engines, is limited to communicating only where there is a Sensor Track, and each Sensor Track is in the $100 price range.
DCS, right out of the box, allows control of conventional engines from all manufacturers. This is an extra-cost option with legacy.
Controlling accessories and switch track is more cost effective with DCS. An AIU (street price around $85) allows control of just about any brand of switch or accessory. Each AIU can control 10 switch track and 10 accessories, 20 devices all together, at a cost of approx. $4.25 per device. However, one must wire the devices to the AIU.
Legacy controls switch tracks and accessories using a variety of add-on devices (SC-1, SC-2, ASC, etc.) at a cost per device that is considerably more than that of the DCS AIU. Accessories and switch tracks need to be wired to these devices, however, the devices themselves communicate wirelessly with the Legacy Base.
There are available FasTrack switch tracks and Lionel accessories that can also connect wirelessly to the Legacy Base, obviating the need for additional devices while eliminating some wiring. However, these switch tracks and accessories carry a premium price, and the switch tracks are limited in what those turnouts that are available from Lionel as compared to what's available from, for example, Ross, Gargraves or Atlas O. Z-Stuff also makes switch motor for Gargraves, Ross and Atlas turnouts that talks to a data wire driver, which then talks wirelessly to a Legacy Base.
Lionel is at present, ahead of MTH as regards the ability to use a Smartphone or tablet to control Legacy engines via WiFi. Lionel has a system of components that may be used to accomplish this already available, as well as both a free app and a $50 app. However, I fully expect MTH to level that particular playing field by the end of this year. While the Legacy WiFi system works very well (I have it installed on my layout), it is, in my opinion, quite expensive. However, I'm rather certain that MTH will do its best to be expensive, as well.
Legacy engines are only available from Lionel and DCS engines are only available from MTH, although many TMCC engines are available from Atlas O and others. However, kits are available from MTH to convert any engine with a DC motor and a flywheel to PS2 (very soon PS3) and kits from 3rd parties can convert almost any engine to TMCC. However, no such kit exists to convert any engine to Legacy.
Two final differences are observations and opinions on my part:
- Legacy is more centered towards putting one "in the cab of the locomotive" while DCS is more directed towards addressing more of all of the aspects of a layout
- DCS is more defined by it's software while Legacy utilizes a lot more hardware.
Somehow, my 2 cents turned into a dollar. It must be due to inflation.