I have single-vent, double-vent and ventless suits and jackets. I prefer double vent. I think most individuals in the forum culture, aside from the diehard "trads," do prefer double-vent. Most American RTW is single vent. As somebody else remarked, most American consumers probably don't give the matter too much thought.
Since Americans carry guns more than most other people, single-vent is better for concealment...or so I have been told by Jerry Ahern, who has been a holster designer and has written often about concealed carry.