DTM 2013 or 2014 are worth buying if cheap as you get to use the tracks for all of the other content and the cars should also be able to be used on non-DTM tracks. Further, you should qualify for a discount when you buy the 2015 DTM experience if you've purchased a previous version.
FWIW, the 2014 DTM experience is pretty damn good - the DTM licence holders are very demanding about their product being represented in the sim and Sector 3 really improved the DTM cars handling and feel between the two releases.
With the 2015 version, two drivers (Daniel Juncadella & Bruno Spengler) have given up their free time to help Sector 3 improve the DTM driving experience primarily because Spengler enjoys driving his DTM car in R3E and has a few active DTM leagues going.
There should be a Twitch stream soon with either Daniel or Bruno soon. Anyway, enough of the sales spiel - the 2015 will be worth waiting for