I like a trailer. When I was going through the panniers vs. trailer debate, my wife settled it by giving me a BOB for my birthday. I'm glad she did. A handlebar bag and trailer have all the capacity I'll ever need. The BOB has plenty of room, a waterproof bag, a durable frame and wheel (mine has easily replaceable cartridge bearings), and a sturdy attachment yoke, I don't trust the special rear quick release required for attaching the trailer (one fell apart on me), so carry a spare. In use, you forget you're pulling one after a while... and I prefer a trailer to panniers in a crosswind.

Towing one behind a tandem makes for a long rig... you won't be making any quick U-turns for sure. Without front panniers, it'll give you way more capacity than you could get with just a set of rear bags. Have fun planning youir ride. Later.