Vacuum secondarys are very good for gas mileage, heavy cars and general performance useage.
Mech. secondarys are great for 4spd cars, very hi po auto trans cars and excellent for the track. Light cars as well.
This is not to say that the list as writin above can not be reversed, it's just best sutied in this manor and easier to do in that order.
The amount of power the car has along with cars weight, intended purpose, cam size, intake style, rear gears and tire size would be good info to make a recomendation on what carb style and size to use. Sometimes, the info will almost dictate what should be the best choice for your car.