Forty litres should be more than enough, but I would never be without a handlebar/shoulder bag on tour, for those valuable and often-wanted items, that I never leave on the bike.
Ortlieb are the best, but Rollers are less convenient than the drawstring-and-hood Bike-Packer design, which is quicker to access and close, especially when you slip off a jacket and simply stuff it under the hood – which also keeps the contents separate from the damp jacket.
Whilst it is true that a roll-top makes the bag utterly watertight, so you can throw it in a river and it’ll simply float, in all my tours I have never seen anyone do that! On a bike, water comes at a pannier mainly downwards and at worst horizontally; Bike-Packers will cope with that.
The one disadvantage of Ortlieb panniers is a lack of pockets for small things like tools you need in a hurry. I’ve added Ortlieb’s (rather expensive) accessory pockets to my panniers. It seems drastic to make holes for the fittings, but they work fine and seal up perfectly – just like the all the holes the bag already has for the rivets and screws securing the rack hooks etc. onto its back.
This was first published in the April / May 2015 edition of Cycling UK's Cycle magazine.