How do the various DUATS differ?
Both DUAT (DTC) and DUATS (CSC) provide the same information. They are functionally equivalent.
You also left one out: Lockheed-Martin operates a web-based briefing service at 
https://www.1800wxbrief.com/ .

Which you prefer to use for interactive access is largely an aesthetic consideration - as a pilot you can sign up for an account on each of these services, so the usual advice is to sign up for all of them and use the one you like better (personally I used to prefer DTC's DUAT interface, but the direct (1800wxbrief.com) system is a now my preference for web-based briefings when I'm not using ForeFlight).



From a non-interactive standpoint, CSC DUATS has an API that can be used by other programs (like ForeFlight) to receive weather/NOTAMs/etc. and to file flight plans. This can be advantageous, however Lockheed-Martin has recently (2013) implemented an XML-based API that lets application developers directly communicate with the automated flight service system, and it is likely that this will be the preferred way to handle this sort of thing as time goes on.