What's the best trailer in the mountains with some offroad mixed in ?
Bob sounds best for single track but Burley sounds more stable.
Will pull mostly with a MTB, but occaisonally with a racing bike.

Single track = single wheel.

I'm a big fan of the Burley Nomad and would use it over a Bob any day - but not on single track.

I used a BOB for many years of touring and liked it a lot. I recently started using the Nomad and for me, I like it better. But for your intended use I think the BOB would be the better choice.

To clarify, the MTB/trailer will probably be used 65-75% of the time on pavement, 20-30% on gravel or cinder and on single track trails no more than 5% (for reaching backcountry camping spots). All the miles with my other bike will be on pavement.
I haven't even considered touring on single track, as most trails around here are either just too difficult or loops. So, with the minimal amount of single track, the decision really hinges on how the Burley will perform on forest service roads and rails to trails. The bob ibex does have suspension, so that's definitely an advantage. Are people actually touring on single track trails ?

I see alot of used kids trailers for sale. Is there any reason you couldn't use one for touring ?

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.