http://www.oldmanmountain.com/

i've done a ton of this stuff.

so this is the deal.

i.e. a BOB trailer is about 17lbs, and obviously another wheel.

look at those OMM racks, both racks combined are around 1200gm
1200gm/454gm = approx 2.6lbs
panniers maybe about 5lbs, if that.

a bike with panniers is much easier to get on and off a bus/train compared to pulling a trailer.

a BOB trailer is obviously really easy to use, simply change the skewer on your bike, attach trailer, and its done.
but its 17lbs (empty).

what it often times comes down to, is what exactly do you have to carry. maybe the size of what you are going to haul would not fit into panniers.