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).