The two wheel trailer will work well most of the time. From my experience with a child trailer on a variety of terrains, here is where it would not work as well.
– On fresh snow, because you dig three trails, one for the bike and two for the trailer.
– If you are riding through chicanes and the like (found aplenty on bike trails in some jurisdictions, supposedly for our “protection“); the BOB will track better and hinge more easily around these.
– If you are riding on a very rough road with 60-year-old pavement, it is easier to find a half-decent track for the bike and the BOB than it is to find 3 half-decent tracks; in spite of some very bad passages around here, I haven't found too many places where it would be problematic.
– If you really want to hog the white line, then with a 2-wheel trailer you'll have to stay a 20 cm away or the trailer's right wheel will be on gravel. Not a real issue because you shouldn't be riding this close to the edge anyway.
– Real singletrack. A well-groomed trail where two pedestrians may walk side by side is quite passable with a trailer. You might not be able to ride around tight curves as fast as with a 1-wheel trailer, but since you are not riding all the time there, it won't be a significant issue.
On the other hand, if you travel on very narrow trails, where you litterally turn around a tree, where the surface is very uneven laterally, or if you ride or walk on catwalks, then the 1-wheel trailer would be better.
As for children trailers vs cargo trailer, I'll offer these points:
– My 2-children trailer uses 20" wheels while the Burley Nomad uses 16" wheels. Does it really make a difference on the trail? I know it shouldn't do one on asphalt. In my case, as I was towing a trailercycle (20" wheel) and a child trailer, it was easier to cope with one size of small wheels.
P.S. the BOB uses a 12-inch wheel; it should, in theory, be more of a problem on gravel, yet BOB owners don't complain about that specific issue.
– A 2-children trailer is 32-33 inches wide, so it's a very tight fit through door frames (getting the grocery inside) and on some narrow trails. A single-child trailer and the Burley Nomad are 25" wide, which is about the width of a cyclist. IOW, if you pass easily, your trailer will go through.
– A child trailer is higher than a cargo trailer. More wind resistance, especially in the 2-children variety. With a strong headwind, pulling the trailer felt like pulling a parachute. For that sole reason, I find it great for groceries or other local errands, but I would not tour with a 2-children (or even 1-child) trailer.
So far, I have toured only with panniers. However, 2 years ago, my 11-day tour with my oldest daughter meant I had a single bike + trailercycle combo weighing 150 lb (bikes, gear, food and water included). This year, for a 4-day tour with 2 daughters on a tandem + trailercycle, I estimate I had around 150-175 lb. Maybe I'll switch to a trailer. But before I buy a cargo trailer, I'll load my 2-children trailer with the gear I carried to compare both riding styles.