FWIW the Streamlight Strion operated on the same piston principle as well, with the piston being the battery stick.
Anywho, I have them both and do have a slight preference for clickie lights - OK a decided preference but I do carry some twisties as well and they do have different pros and cons.
Twisty.
Reliability - I think that a Peak LED Solutions twisty light is about as foolproof a light as you can have - I've had an Arc AAA fail on me but never a Peak. I'd add the old CMG Infinity into that group as well. I do think the basic twisty design is the most solid and foolproof overall.
Size - for a keychain light or any light where size is a factor a twisty is the smallest and most compact option, although the PD design is getting there.
Clickie.
Utility. But for any multi level light a clickie quickly becomes a better solution for me - easier to activate and effectuate multi steps to access different states. The Novatac/HDS light is the top of the food chain in this regard for me - the effective and reliable clickie allows you to move directly to 4 different levels from any of the other levels, something you just can't do with a twisty (and I have a Liteflux LF5 and LF2 so I can testify that the twisty is not as easy a method for programming).
Easier operation. Gloves, one handed etc... it is easier to use a clickie to me.
The reliability of a clickie is highly based upon the maker - my HDS and Novatacs have been very reliable, my Dereelight CL1H can be a bit finicky at moments.