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 ?