Hmmm...interesting discussion as my boys use the 11 and I have no experience of the Green bus.
On the 11, being a public bus, there are often youth smoking (and not just cigarettes), occasional fights and threatening behaviour, including people refusing to pay their fare, and overly amorous couples (to make clear, these are not CH pupils, but the public!), and it is usually overcrowded; the boys often stand.
In fact sometimes they have to wait until the second or third bus after school because they are too crowded and the driver won't open the doors.
However the cost is many times less than the Green bus - I was told that the fares were rising to £1000 next year - if anyone can confirm?
Fortunately they are both still in one piece and have never complained (except when ds1 was punched at the bus stop) and seem to relish their independence.
I suppose you could say that the best transport would be mum's taxi, but is that really going to help your child? As children grow, they really do need independence in so many areas of their life; not least to become independent learners too.
We are about to move house and I was googling how to get rid of the fish pond there (I have small, mad children) when I came across an article suggesting that the best way to keep a child safe around open water was to push them in yourself one day (and help them out, obviously). Now before you call children's services, I am not planning on doing this (and I'm still getting rid of that pond, as I have no time to maintain it) but the concept was interesting.