Depends on how high you intend to go.
I have found the best type to buy is the 4 meter double section ladder from wicks. Its cheap and you can take the sections apart. If for instance you want to go 2 levels i.e ground, 1st, 2nd then this ladder would just about do the job. If you are on a normal domestic house then you just take of one section and use that all round.
It takes a while to get all your windows figured out this way but I have found it better than getting a double section 3 meter as I hate keep lifting it up and down on each window.
Steveyboy