Actually, it’s not that one’s harder than the other, they just have different focuses. The pistol is more like a traditional squat, while the shrimp is more like a lunge. The shrimp is more hamstring dominant, while the pistol is quad dominant. But both variations will definitely hit the entire leg. I personally find pistols more difficult than shrimps, and in general I find the pistol to be a better overall fit as a standard leg exercise. Unless you’re talking about the jumbo shrimp squat, the pistol wins out on range of motion. Furhermore, many other skills, such as bridges and levers, require a strong posterior chain (including the hamstrings). Since few bodyweight skills focus strongly on the quads, the pistol is more appropriate as a default leg exercise. But the shrimp is a great ancillary exercise!