I prefer panniers as I see the trailer as just one more big thing to deal with. There are always situations on my tours where a trailer would be a real pain - needing to lift the bike over a highway guard-rail, getting through a British stile, etc. I've toured with my bike weighing over 100 pounds (not recommended, but I even managed a century [double century?] one day), and I still think it's easier to handle than a trailer would be.