40 volume is stronger and it works faster than the 30 volume, but it also damages hair a lot more easily. if your trying to do highlights I would go with the 30, but if you're doing all over color I would go with the 40..and make sure your hair is a very pale yellow before you wash the bleach out or it will be orange.
The developers. 40 volume is a lot stronger, and processes faster then 30. And I wouldn't use 40 volume, unless you have colored Black hair.