John Palmer gave a good explanation on Brew Strong way back in the archives. It was probably in the April/May 2010 shows on packaging/kegging. From memory 5-6' of the thinner 5mm line was good for a normal 2.5 vol CO2 ale at 4C (which is what I use). You want more resistance from a longer...
Flexovit ones are good quality, that's all I use. The thin wheels cut easier as there is less metal in contact with the wheel, less heat, less discolouration of the metal. As you're new to angle grinders don't be tempted to clean up the rough cut edge with a thin INOX wheel as any side...