I prefer the hard shell saddle bags that mount/bolt-on to the frame… I find them durable and very secure to ride-with and leave-out… I have factory made ones on my K… I have seen aftermarket products that are of same quality if not better… I'd recommend a similar setting if you are looking for that storage capacity and security… I usually keep them mounted most of the time… They are easy to access and remove…
I am entertaining a top case option too… It will be more geared for the passenger’s comfort rather than the extra storage… Something for the passenger to rest-on while on long rides… The bracket setup would be my pick… With a 40L or less, depending on how harmonious the case will look with the overall bike… If it can fit an extra helmet, that would do it for me… Anything too big and you got yourself an ugly view…
Of course there is always the tank bag option… I have a bags-connection with quick-lock for SW-MOTECH gascap mount… Its the best tank bag setup I have seen so far… I couldn’t be any happier with it… You can vary the size as you see fit… I have the option to expand from 15 to 22L…
If you haven’t checked it out already, Twisted Throttle has a ton of luggage options… Brows though their catalog when you get a chance… You can download it on their main page...
http://www.twistedthrottle.com/