Join Date: Sep 2007
Location: Mountain View, CA, USA
When I've done long tours I've usually ended up with both.
The trunk wins IMHO because it's waterproof and lockable, and I end up wanting both. And then there's always some extra stuff. (But then I've done camping tours, so I'm carrying a tent and sleeping bags... add a rack to the trunk and you can strap extra bags to it.)
A bag is probably plenty for a hotel tour or shorter trips where you have the flexibility to wait out bad weather.
Trunks are more convenient to get at things while they're on the bike. I've always ended up having to take bags off the bike to get anything out of them.