1. From what I've read, I believe the only appreciable difference to the user is cost. Several doctors have advised me to not use any with pressures above 20-30 mmHG without their guidance. And that some of the sport specific ones cut off above the ankle, which may lead to blood pooling below that. There don't appear to be enough studies yet determining the effects of any brands or styles to make a definitive answer.

2. This probably goes for other sports as well. I havent seen enough evidence that wearing them while cycling, running, or doing any other sport helps performance.