Well, I have NovaTac 120P, which is essentially the HDS UI, and I have the ZL SC600. It's really kind of tough to pick one over the other, as they each have unique qualities which might appeal more to one person or another.
What I miss on the Zebralight is a function which I use constantly on the NovaTac, and that's press and hold for momentary high, either from off, or any level. What's cool about that UI is that I could also set it up for momentary strobe from either off or any level.
That's where the HDS/NovaTac UI shines........it's programmability, letting each user decide how they want to set up the UI. Last level used memory, or not..... your choice. Momentary on, or not...... your choice. A quick click to turn on high, or low, or any level in between....... your choice. Have the light turn off after 10 minutes, or not...... your choice.
However, after using the SC600 for only a week, I've come to seriously appreciate it's simplicity and usability. A very well thought out and implemented UI.
Honestly, what would be the best for me would be to incorporate the HDS UI, but in the SC600 form factor, with that lovely, deep side switch.
So, both UIs are excellent, for different reasons, and what I think will end up making more of a difference is whether you want the end switch or the side switch. Personally, I'm sold on the side switch as implemented on the SC600.