There are definitely a decent amount of questions added to this version. Dr. Romano definitely recommends getting the newest version, as do a bunch of other members on the forum. But honestly, it's really up to you.
From what I saw in SDN the 2015 have more problems added. I don't have it on me now but I'll gladly give you the breakdown of how many problems are in each section.
I used destroyer 2014 and did well on the DAT (25AA/25TS), I took the test in May. The extra questions are probably helpful but if you learn the 2014 frontwards and backwards you can knock this test out of the park!
From what I saw in SDN the 2015 have more problems added. I don't have it on me now but I'll gladly give you the breakdown of how many problems are in each section.