I took have the 20+10+10 combo.
I can fit all my 'civilian' clothes into the 20 for a week. I do have a system of packing that involves using roll-up-vacuum-bags to take the air out and 2 TT 'cubes' (
http://shop.touratech.co.uk/touratech-cube-red.html) that fit inside the US20 perfectly.
There are some that cast a suspicious eye over packing things in bags , to go in bags.... But I can get more in this way than I can just stuffing stuff in, as I say, I can pack a weeks worth of clothes into the US20.
The really goo d thing about the US range, you could be a US20 and a US10, if it's not enough, then go buy another US20, or a US30 - they all attach together one way or another.
I even made a 'strappy' carrier to carry a 5L fuel can, attached to the US20!