I have the 20+10+10 and love it. On short day trips, I always have a 10 on my bike. I haven't used the 10 as a tank bag, as there is other stuff out there that works a bit better in less than torrential downpours.
I don't do anything long distance more than 5 or 6 days and the Kriega system works for me. Some think it is a little pricey, but it is worth every dollar. Extremely well engineered with many thoughtful details.
Top notch gear designed and marketed by real deal motorcyclists.
The US10 is awesome. I love mine and it's perfect for essentials including pants liner, jacket liner, tire plug kit, rain gear, wallet, phone, etc. It fits quite a few things without looking out of place on the bike. The fit is perfect in my opinion. If u go more than a couple days I am sure the US20 is good. Not sure the fit of the US20 but Kriega is makes some awesome stuff.
I have a kreiga 20. There are a couple of metal tie down points under the seat. If you position them right the bag will sit on the passenger area of the seat. Just becareful the tie downs don't interfere with the locking mechanism for the seat. I've had that happen a few times.