I use short gaiters in summer and switch to the long ones when there starts to be a threat of snow. Ditto with footwear--it's trailrunners in summer, but back to boots by late October.

My main problem with the knee-high gaiters is that they don't come in my size, which would be a couple of inches shorter than most knee-high gaiters. I end up with a wad of gaiter behind my knee! I certainly wouldn't try to use the short gaiters in more than an inch or two of snow, though.