Differences are as follows:
HDS has better regulation/electronics, 3mm glass lens which is sandwiched between O rings for shock proofing (Ntac=polycarbonate), has 4 programmable levels (NT=3), fully potted electronics, springs on both positive & neg terminals for better protection, better CS, stronger threads, no flickering, is manufacture-rated for EMP, has a thicker SS bezel, better and more durable electronic switch, and it is the the generation after the Ntac. Also, I believe the base HDS is cheaper than the 120p.
The Ntac is a good light but the HDS Clicky is simply a far superior light. The only reasons I could see someone choosing the Ntac is for the slightly shorter length, or if they preferred the look of the NT, or if they needed a remote tape switch. The cheaper Ntacs (Storm, Whichita, Spec Ops, etc) are made in China and are only type 2 anodized as opposed to being HA III. They are also not programmable from factory but can be after applying the 250 click Easter egg.