Copics are made with alcohol inks. You cannot color onto a stamp and then stamp. they are made to draw with or to color in designs. They will also color ribbon, paper and other embellishments. They bleed through to the other side of the paper.
They are very expensive, but if you practive, you can achieve some very very nice shading and blending effects.
You can learn more by reading these two blogs
www.ellenhutson.typepad.com and
www.debbiedesigns.typepad.com
If you just look around those blogs there is a lot of info on how to use the markers.
SU markers are water based and generally don't blend as well. However, a lot of folks have achieved some very nice blending and shading with the SU markers. Again, it is a preference thing.