I only have double and I think they're decidely better. I think at this point nothing so basic as vents can be described as conservative or not. I've seen very fashionable jackets with all three types and very stodgy ones with all three. At the moment, double is very trendy though. I don't think that none-vented jackets are more slimming. Or at least, the brands that happen to make non-vented jackets are the ones that also have fuller cuts and most people I see wearing them are also larger. I haven't seen non-vented jackets from guc/ysl, Hedi Slimane, Costume National, etc. Double is the best though. I prefer really high vents, but they draw too much attention, and thus don't befit a gentlemen. Two vents look flow better while you walk. Similar to not-buttoning the lower button of a two button jacket (something MAnton convinced me was more than just a rule in another thread).