Single vents - for tweedier sportcoats and American sack suits only. And even then I don't like them. They aren't attractive and they aren't very functional unless you are riding a horse. No vent - probably the best looking and definitely the least functional option. Non-vented coats tend to wrinkle something awful when you sit down with the jacket on. Double vents - the best choice. They lengthen the line of the leg, your rear stays covered when you put your hands in your pockets, and they wrinkle the least when sitting down. The only problem with double vents is that Italian companies cannot seem to cut the vents so that they hang properly. They need to examine the double vents on a newer Oxxford suit and copy the technique.
i agree with mr. harris. eleven years ago i went to a party with my uncle. i wore a non-vented suit and he wore one with side vents. at the time vents were not something i considered when buying a suit, but after i saw him put his hands in his pockets, and how it didn't ruin the look of the jacket, i realized side vents were the way to go. many people will say the non-vented is more elegant, but that's for you to decide. i don't think it looks elegant to have your jacket bunch up when you sit down or put your hands in your pockets. stay away from the center vent though.