There are advantages to both but at the end of the day - the choice is yours.
Manual: The majority of people will learn in a manual car. The main reason for this seems to be that once you have passed your test there is a larger choice of cars.
There is also the question of price - manual cars are slightly cheaper.
Your licence will enable you to drive an automatic and manual car.
Automatic: There a few reasons why people decide to learn in an automatic.
Some pupils find learning to drive difficult - if they don't have to learn how to use the clutch and gears, it becomes slightly easier.
Some people don't see the point of learning to use gears and are quite happy to drive an automatic.
On the flipside, when you pass the driving test you'll only be allowed to drive an automatic car, although your licence will act as a provisional licence for a manual car.
When buying a car - there is less choice of automatics and they're slightly more expensive.