The most cost effective way to order is to get 30m and cut it to the length you want (as long as you're not discarding too much!)
It just depends how much trailing pole hose you want sticking out of the end of the pole when it's fully extended, most people want 1m - 3m I would estimate, but then more people are considering using pole hose for the complete length too.
Our hose is quite different to others you may have used though (
reviews here), so you might want to stick with the 10m and see how you get on with it. If you really like it, you could get more later?
Paul