Class 6 is a very good speed, and the best compromise price/speed in my opinion.
You could go for class 10, but the prices go up considerably, and let's just say that class 6 is more than fast enough to handle recording HD video in a camera, so is perfectly adequate for an ereader.
I've briefly tried a class 2 in my PRS600. To be honest, In comparison with the class 6 I'm using at the moment, I can't really say I noticed any difference at all.
Memory Stick duo is the exact same product, but a different shape: you'll never be able to use it with a non-Sony device and you'd be rewarding them for a bizarre and unnecessary corporate money-grab.
Last edited by Richey79; 11-16-2010 at 10:51 AM.