Should I Use a High Pass Filter (HPF) or Low Pass Filter (LPF) to Enhance the Features for Image Splicing Detection?
Since you're after a filter which emphasizes abrupt changes you should use High Pass Filter.

The issue is you'd be also sensitive to noise.
Hence one way to do it is to apply High Pass Filter on slightly blurred image (Which actually results in a Band Pass Filter).

One easy choice would be using the Gradient of The Gaussian Filter.
Why is it?
Because it is easy to calculate (Analytically) and efficient to implement because of its separability property of the filter.