The two models are quite similar in look and operation with some small differences...they both have "clean" slides, that is no external safety or decocker, they both have shaved external hammers, (although you see just the rear of it), and they are both basically DAO pistols.
Now, while they are both DAO, they achieve this in slightly different ways, first, the D model is a true DAO pistol with the hammer being independent of the slide and the trigger pull is responsible for the entire rearward, (cocking), movement, of the hammer and it's release. This also allows double strike ability.
Where as the "C" model (constant action), is something of a hybrid, while still a DAO, it shortens and lightens the trigger pull by allowing the slide to contact the hammer as it cycles leaving the hammer in a safe, (full auto safety is engaged), "half cocked" position. When the trigger is pulled the hammer moves rearward to the fully cocked position and releases just like the "D" model would. As the round is fired and the slide cycles it moves the hammer back to the "half cocked" position again ready for the next shot. In effect the hammer doesn't have to move as far to do the same job. The result of all this is a nice light, smooth, and short trigger pull for a DAO pistol while maintaining the safety factor!
You do give up double strike ability with the "C" model