Simon -- Are you riding 2-up? What do you want or need to carry in your cases for "long" trips? One of the most useful things I've read over the years about packing for longer trips (and therefore the associated luggage you will need): treat a long trip as simply a series of 3-4 day trips. I've taken that advice to heart, and it's allowed me to focus on slimming down the stuff I need, and therefore the luggage space I need for said stuff.
Also, think about how you'll use the luggage most of the time
. For me, long trips at this stage of my life are the exception -- I make roughly one 7-10 day trip per year, with multiple day or short weekend trips. Since I use my luggage 99% of the time for daily commuting and around town stuff, I chose accordingly.
My current setup:
Top Case: 26L Storm iM2400, hard-mounted.
- Hepco Becker Lock-It racks
- Caribou Cases mounting kits for the bags
- 19L Storm iM2370 cases
I use the top case and small side cases just about daily for commuting & around town use. Their 64L of total space is adequate for grocery shopping, carry out, and other errands, and the side cases are nice and narrow (a couple inches narrower than my handlebars) for moving through urban / suburban traffic.
Client Site Visits
On the occasions I need to travel to client sites, my boots go in one side case, the pants in the other, helmet locks to the bike upright so it doesn't collect water if it rains, and I wear or carry my jacket inside. (My jacket is a Motoport Marathon with Quad armor, which won't fit in all but the biggest cases. I'm not willing to keep a massive case on the bike for the relatively few times I need to do so. If I need to stash a more normal jacket, like my Darien, it would fit in the top case.)
For day trips rather than daily commuting-type stuff, I pop off the side cases and
the side racks and run with just the top case -- takes all of about 60 seconds.
Over the years, I've found myself slimming down my luggage capacity and focusing on packing lighter. Wish I'd done so sooner. If for some reason I need gobs of space -- haven't yet in ~2-3 years with this bike, anyway -- I can swap out the side case hardware to a pair of 40L SKB cases (they came with my Caribou setup when I got it 2nd hand from my meathead brother), or load up a waterproof bag and strap it to the seat.