I have both. These packs work best if stuffed full. For a simple day-trip, I prefer the US-10. I can stuff a rain suit and extra pair of gloves in it, or camera stuff, ect. I don't notice it behind me when moving around in the seat.
The US-20 is good for a several-day trip where you want to bring along extra clothing, guns, a laptop, ect. I hauled an impact wrench in it once, lol. Packed full, I do notice it behind me when riding aggressively.
Get both - you can attach the 10 on top for extra capacity. Each has it's time.