i tried the smaller one on for size last time i was up at john's, and i liked it, fit well with the straps, but it was empty. holds about 18ltrs, a bit more if filled to the brim.
it comes with a great trigger attachment to use with a pole, and it only has one speed, apparently its 'just right' flow.
the 25ltr one has a flow controller.
the switch to turn it on is in an easy to reach place on the side.
sold out at the moment though, but i'll have one when he gets more stock.