My preference is a two wheel trailer... and it really does get down to personal preference. I've used a Radical Designs Cyclone trailer (available from
www.biketrailershop.com; trailer is made in Holland) on 5 tours over the past 2 years including last summer's 3,600 miler from MT to Maine, and it has performed well.
My experience is that two wheelers are, in fact, more stable and don't "steer" the bike. Also, a single wheel setup puts a significant load on the rear dropout/triangle (since the load is distributed between there and the single wheel). A properly load-balanced two wheeler has a tongue weight that can be measured in ounces, and the rider often forgets it is there (excepting going uphill, of course).
One disadvantage of a two wheel trailer is that, on narrow roads (or off-road) or roads with a limited shoulder (like Highway 2 in Montana and ND), the trailer can get dangerously close to approaching traffic. This is particularly true if the shoulder is narrow and there are rumble strips or other obstructions. A single wheeler (like a BOB) tucks in nicely behind the rider. Still, this is the only advantage I've seen.
As to specific trailers, the Radical Designs Cyclone has a waterproof bottom (which the Burley Nomad doesn't have) and with the rain cover in place, is virtually water tight on the inside. Not so the Nomad. The Cyclone also is lighter (albeit smaller) and has a simpler mounting mechanism. It also features two wheel positions- one when connected to the bike and another when used in airports/etc. like a large, wheeled piece of luggage. It is, however, more costly than the Nomad.
I've had to make some adjustments to the wheels/axles on the Cyclone to reduce side to side motion due to excess play, and the frame in my older Series II trailer is coming loose at the point where the trailer arm connects to the frame. I am, in fact, now in the process of replacing that frame with the more sturdy, redesigned Series III frame/hitch.