My trigger preference depends entirely on the purpose for the double gun.
I much prefer double triggers in the field because having separate triggers for each barrel allows for instantaneous choke selection, by simply choosing which barrel to fire. Double triggers also offer redundancy in the field - if one lock fails, the other often continues to function independently.
Conversely, my competition U/O is equipped with a single trigger. In competition, the more open choked barrel is always fired first, followed by the tighter choked barrel, if required (my game of choice is International-style Trap). With only 2 minutes grace to resume competition in the case where a trigger fails, redundancy in competition guns is better achieved through replaceable trigger groups.
I agree with Hunter, that single triggers are inherently more complex. However, most trigger systems in use these days have been sufficiently tested and improved over time that failures of both types are the result of manufacturing defects or improper maintenance. With the exception of the poorly executed Savage 311 design, I've not encountered issues inherent in either category.
Like many aspects related to gun selection, the intrinsic advantages of one design over another are often inconsequential, if the shooter has difficulty adapting to a different style. For my part, I ALWAYS use double triggers in the field and ALWAYS use single triggers in competition. I never have to think about what I have in my hands because the habits for each situation have become instinctive. It helps that all of my field guns are juxtaposed and all my competition guns are superposed.
