Same line, same QR2, so your wheels carry across either way - this is purely a torque-and-chassis decision. The ClubSport DD+ adds 3 Nm to reach 15 and brings a build-quality step up that was a real jump from the CSL DD line, while the DD 12 is the cheaper rung that scores a touch higher on value. Twelve Nm is comfortable for most driving, so the DD 12 covers a lot of people.
The DD+ earns its $200 if you want the extra headroom and the sturdier chassis - it is the base Fanatec used to launch the QR2 era at the upper-mid tier, and it feels the part. For a buyer who is happy at 12 Nm, though, the DD 12 keeps you on the same platform for less. Either way you are inside the Fanatec ecosystem, so the upgrade path stays open.