One big advantage that soft bags offer is usually the ability to quickly detach and carry the bag with you, so you can bring along your goods in one fell swoop instead of having to open the box to remove them as most hard cases will be more "hard mounted" to the bike or rack. There are some exceptions of course, but as a whole this is kind of the general rule.
I have been eyeballing a bag like this more and more as it offers a decent amount of space, is also expandable for even more space, but isn't overly huge. It's water resistant, but also comes with a rain cover if you get caught in serious weather, and it connects and disconnects with 4 quick release buckles and has shoulder straps to carry/
https://www.amazon.com/Motorcycle-Ba...NsaWNrPXRydWU=