I personally love the Moose Racing bags but if you can find them you better grab since I believe they are discontinued and getting hard to find! They are phenomenal products with an internal structure to help them hold they're shape, they are also expandable to nearly double the height you see and come with rain covers! They also have a strap mounting system on top to add additional items like jacket liners or hydration packs for easy access.
The large front bag I use exclusively for my "tool box", spare tubes front and rear, air compressor, spare fasteners and zip ties etc... The rear one is the one you mentioned Derek and I use it mainly for extra hydration bottles and snacks and misc item I may pick up along the journey like the time I filled it with apple butter shortly before the beloved restaurant in Cottonwood Falls went out of business, she made THE BEST homemade apple butter!!
Moose also made a larger bag that basically was the two I use combined I think? There are tons of options out there though, too bad we can't get hands on most times to evaluate them! I actually first saw the Moose Racing tail bag you linked on the back of a couple bikes at a Stampede including Phil Denk's KLR so I looked them over and made an informed purchase..