If those are your only two choices, then it’s a no-brainer: Shifter Kart.
Even if the DD2 is potentially less maintenance than a 125 shifter, and even if a you would only need to deal with 2 gears instead of the shifter’s 6, the DD2 is such an oddball class of kart here in the U.S., you could potentially run into problems such as finding other DD2 racers to compete with, and resale value of the kart if you want to sell it. The second point may be even more exacerbated, now that the importer of the DD2, Maxspeed, has just recently closed operations. That could also lead to parts availability problems in the future.
The DD2 is direct-drive instead of chain-driven, which every other racing kart out there is propelled by. While that may seem like a plus, there’s a disadvantage in that a DD2 requires a unique chassis different from every other kart class and chassis now racing.