QTC has its ups and downs like anything else. On the up side, it allows you to get something less than full power from your light. I won't go so far as to say it's "infinitely variable" because it doesn't consistently and smoothly ramp up and down like a light with a magnetic control ring. Sometimes it can jump up or down a little bit as you're compressing or decompressing the QTC. What that means in use is that sometimes if you want a nice low output, you begin to tighten the head and all the sudden it's too bright. So then you have to loosen it a little bit and you may again go too far etc.
All that said, if you just tighten all the way you get full power just like a non-QTC model. Same with loosening. Loosen all the way and it's off. So really, it behaves just like the non-QTC if you don't need the brightness adjustment.
So what's my recommendation? Get the QTC. If you know you want a Peak light anyway and you're open possibility of having a single mode light, the QTC just gives you a little more flexibility. You can operate it as a single mode when you need it to and in situations where you want a lower mode, you have that option. The beauty of the QTC design is that it doesn't
add any mechanical or electrical complexity to the light. It has the reliability of a twisty with a modicum of adjustability.
Cheers!
-G