I like both, but if I had to choose one, I'd go for the twist on. In my experience, the clickies just aren't as durable as a twist on light, there's always some moving part that's going to wear out or break. Plus, you don't have to worry about accidentally turning it on in a bag or anything. With a clicke, there's all different lockout mechanisms to prevent accidental turn on, so really sometimes it's not that much quicker to turn on the light than a twist on. Simpler is better.
Also, it depends on the size of light you're using it on, a twist on light is fine for small lights, but cumbersome on larger ones. But a tailcap clickie isnt' that useful on a smaller light for me, because when I'm holding it, it's usually a waist level, which means the lights is pointed backwards. If it's something like a minimag sized light, it just doesn't feel right to hold it to up at shoulder level.