One of the huge sells of ShadowPlay is that it doesn't place extra burden on Windows to capture the screen every frame, instead it directly captures the actual framebuffer (and encodes the frames) directly on the GPU, without having to switch or bottleneck any rendering code paths. It wouldn't surprise me if people who swear by ShadowPlay can tell the performance difference compared to software screen capture.
Basically it aims to replace the need to for an additional out-of-band capture card (those can get very expensive, especially if you want 4k60).
I don't think you're going to be running into much performance difference between the two. People say GameCapture already grabs the frame buffer, but I can't say whether that's true or not.
Most likely, different framebuffer - ShadowPlay grabs the exact frames sent to the physical monitor while GameCapture intercepts it in software (most likely anyway, if it doesn't use the Nvidia-specific APIs that ShadowPlay does). The former doesn't require inserting any extra steps into the rendering pipeline, so it's generally faster (sometimes by a lot).
Basically it aims to replace the need to for an additional out-of-band capture card (those can get very expensive, especially if you want 4k60).