There's 2 versions. The NH-D15 is slightly older, but, is also the better of the 2. The NH-D15S only has the center 140mm fan, and has an offset base for better ram/gpu fitment. It, however, is only 3-4°C behind the original D15 at max ability, so mostly unless intense cpu loads are run, it'll be 1-2°C behind.
So depending on if your gpu has a backplate, or the pcie slot sits abnormally close to the socket and you have low profile ram, the D15 is better. If you may think you have issues with Ram or gpu, better to opt for the D15S.