There are certain applications where both work great, and where both work horrible(DLC vs Nitride).
DLC is applied at roughly 200 degrees celcius, and as such, won't affect most heat treated steels. Nitride on the other hand, is done at 530 degrees celcius, and will destroy most heat treats in common steels used in the industry.
Let's take a 9310 bolt for example and look at what will happen to it using each method.
DLC-The case hardening and core hardness will remain fully intact. You will be adding a very thin coating of an amorphous carbon that not only repairs itself to some extent, but holds onto oil quite well. It is extremely hard, just short of the hardness of diamonds, but it is extremely thin. This coating works great in all tribological environments, such as it would be used in a bolt carrier. The caveat with this coating is that it is purely a line of sight coating so it won't necessarily get into all of the bores perfectly. Due to it's amorphous structure, it is also quite good at corrosion protection where the coating is. Adhesion of this is also extremely tough and will never chip. However, due to the thin nature of it, it can be cut through and removed. In a normal use situation, this would never happen. This coating can be used on most any conductive metal.
Nitride/H&M/QPQ- If this is done on a case hardened 9310 bolt, not only will the case hardening be destroy, but the core hardness will drop as well. This is due to the extreme temperature used during the salt bath process. The nitrided surface is significantly more hard, but 7 times thinner than a standard case hardened bolt. The surface conversion process actually does add a little bit of dimension to all surfaces, but not enough to generally affect anything negatively. This is also not a line of sight process so all surfaces are "coated". It's corrosion resistance is phenominal, and even better when oiled. This can be a great "coating" if used on the correct materials, but can destroy the incorrect materials.