I think there is a big difference between the UI used for operation for the UI used for programming in the more complex lights. The first two options in the poll seem to focus on memorizing the UI to program the most complex lights. Any way you tell a light what to do is it's interface - down to something as simple as a twisty switch with two states, on and off, and zero programmability.
To illustrate where I am coming from, let's look at 3 different lights set up as 2 stage UI's in operation - the NDI, Surefire L1, and the LF5XT (setup with 2 modes ignoring that it still has 100% and strobe always available). Note I don't own any of them, although I have looked closely at all of them for my "wish list"; if I get the actual UI wrong it's still a demonstration of my thinking about UI's. Alternately we can think of them as light A, B, and C with notional, but possible UI's.
Surefire L1 - Progmramming UI is the ultimate in simplicity - you can't. It comes with 2 preset levels. You basically have 5 states for the light (momentary low, momentary high, constant on high, constant on low, and off.) In operation the UI is controlled by the two stage momentary switch for levels and twist for constant on. Pretty simple in operation using it's 2 controls (the twisting tailcap and momentary button).
NDI - Programming is simple, but limited in what you can actually program (user mode light level). 2 controls to enter the same 5 states (bezel and forward clicky button). Also pretty simple in operation, just different controls that you may or may not find work better for your needs.
LF5XT - Programming is the most complicated of the lot because there are so many options, but you can set it up to be a 2 level light. All controlled at the tail button so more clicking but not twisting. Not complicated to use as a 2 mode light once set up.
All three can be used as relatively simple 2 mode lights. Different UI's in operation for different user preferences and uses. Complication of programming varies quite significantly between the 3. That complication buys you function if you can use it though.
To answer the poll based on two different UI aspects, the one in usage and the one for programming:
-UI in usage is critical. It's how I make the light do what I want/need without distracting me from what I am doing. It gets used thousands of times in the life of a well used light.
- Programming UI is less important to me. It should be as simple as possible for the functions it offers, but I am okay if I need to have the manual in front of me for less used settings. I don't expect to use it often. I understand that extra function adds complexity. If that complexity lets me adjust the UI for usage by making the light work better for me, then the hassle is more than worth it.