Take a close look at the specs of the two;
The input resistance of the HT is a lot lower, 4k vs 44k.
This is then reflected in the 'On Voltage' the HT is 15vDC@3.5mA vs 10vDC@0.23mA
That's about the guts of it. The HT is more leakage tolerant, if you have a prox switch that is going to leak a bit of current in its off state, the HT will be the one you need as that leakage would probably turn on the SNAP-IDC-16.