Well the gray comes in a firm version which makes it excellant for sculpting and holding it's shape without getting messed up too bad from handling. Alot of times fresh super sculpey is just way to soft to use very well without leeching. And if you over leech you will take out too much plasticiser in the SS and it will become brittle and unusable. I learned that the hard way. The gray also makes seeing detail alot easier and just looks very nice.
You can tint super sculpey with sculpey III but that stuff is usually very soft and once it's mixed you may want to leech the clay a bit to make it firmer. SS is cool though and if you have no problem mixing then go for it.
Price wise, If you buy the gray in bulk 6 blocks you get each bar for only $8.39. My local craft stores all sell regular SS for 9.99 a pound and then you have to buy some sculpey III for a couple of bucks to tint it. So for value it's alot better for me personally to go with the gray compund. No mixing colors, no leeching, no fuss. Just sculpt.
hope that helps.