A:Unlike panniers, trailers keep your load off your bike, reducing stress on its frame and keeping its tires from deforming and causing more drag. They’re also more aerodynamic, and for really big self-supported trips, they allow you to carry a huge amount of gear. (Some would argue too much—there's always room for one more thing in a trailer.)
The downside is an added degree of mechanical complexity: another spoke to keep an eye on, another tire to go flat, a connection to the bike that has to be managed. Still, for my money, a trailer is the way to go.
For years I’ve pulled around an older generation BOB Yak, which I love, although it isn't nearly as sleek as the current Yak Plus model ($359) which comes with its own dry bag. It's rugged, connects to your bike securly and safely via an add-on rear skewer, and can really haul a load. And the Yak's single-wheel design, which lets it squeeze down narrow single track, is a major advantage if you plan to travel off road.
One last thing to note: trailers makes handling strange. Before you head out, spend some time riding around in a safe environment. And when you're not riding, the combined length of the bike and trailer means you'll need to find something that both can lean against.
Bike touring rocks!