Hey =) Well, it really depends on the hair you have right now, and what you're trying to get it to, and what product you're using. If the product is professional, they should have small instructions printed on the packet that suggests a volume of developer. If I were you, I would choose the 30 just to be safe(if those are you're only choices). If you can get ahold of a 20, that probably be even better. I use 10 when I'm toning.
Use 30. 20 or 10 will burn your hair off.