My SureFires with clips are a C2 (long clip) and L1 (short clip). From my experience, the short clip is stiffer, while the long clip is, well, longer.
Myself, I like to clip my lights to my belt if possible. For this, the long clip is nicer, because it fits over the whole belt. The short clip only goes part of the way over the belt, and I've had the L1 topple off a few times (I quickly learned to tether the lights to my pants, so it's no longer much of an issue). That said, I think I'd still prefer a long clip even if I were clipping the lights to pockets, as it allows for more space to slip before they fall out.
I think in the end it's probably more of a preference thing, but conceivably one could be better in one situation or another.
So, my preference is the long clip, as I prefer the length, and don't need the little extra strength of the short clip. Also, I prefer the look of the long clip. Just a personal preference there, for sure.