The thing with leaflets is... I believe every thing you can say on an A5 leaflet can be said on a DL size leaflet (which is the same size as a compliment slip) but its 33% smaller so is cheaper, you can buy 10,000 for just £12 more than the cost of 5000 A5
They are easier to deliver as well, it's a bit hard to explain but you can carry 150 in your hand, as you walk betreen house you lap the next leaflet over one end of the stack, open the letterbox with one hand and push the stack through the letterbox then withdraw it, the lapped leaflet drops through landing perfectly on the floor with no creases.
You don't get your fingers bitten off by dogs or scrape them on sharp letterboxes. This is remarkably quicker.