The 'ventless' coat (or, if you prefer, 'jacket') is believed to be the most formal option. This is why dinner jackets and the black jacket worn with a black waistcoat and striped trousers - now worn in the UK only by a minority of (elderly) barristers and some hotel staff - are traditionally ventless.
The ventless coat is sleek and, most women agree, more sexy than its vented counterpart. The disadvantage is of course that the skirt tends to ruck when the wearer plants his hands in his trouser pockets (when the coat is fastened). I also think that twin or double vents lend the illusion of greater leg length.
Another problem to watch out for - and it is this which is more likely to date the coat - is to avoid studiously any ventless coat made in the mid-80s to early 90s (or one following that trend): the notch of the revers tends to be very low. Not only is this dated, but it compromises the 'authority' of the suit.