For those who are interested, I now have in my possession both the Cabin Max Manhattan and the Metz (for Ryanair, but they both come in the Easyjet size) and have done a 'practice pack' with each to decide which one to keep.
Both seem decent qual and are surprisingly roomy, but the Metz has won. This is because a) the front pockets are much better - more of them, more accessible, and lots of organisation potential, and b) as slummymummy24 says above, the Metz will also work as an everyday backpack on holiday (in a way that I don't think the Manhattan quite will for me). The Manhattan does have a laptop pocket at the back but I don't need that.
The Metz also has compression straps which make it more compact once everything's in, and I managed to get it still to measure 40 x 20 x 25 (though see StaringAtTheWater's proviso above).
It also comes in some nice-looking colours. I have the 'petrol blue', which as I mentioned above turns out to have a secret pocket at the back, which apparently they don't all and you can't really see on the website which do.
Let's face it, there are lots of decent backpacks in lots of brands (inc some really nice ones that people have mentioned here) of which these are but two.
Well done if you've got to the end of this post without glazing over.