You take guard so you know where your stumps are when you are in your stance. This is important because if you know where your stumps are, you know what balls to play and what balls to leave when you are defending.  

Players fine tune this by taking a guard on a particular stump to suit their preferred scoring areas. 
 

Leg stump (or 1 Leg)  

Players that take leg stump are generally players who like to score their runs on the off side. It is easy to go off sided from this 'inside out' position.

The down side of taking off stump is that you are stepping across to the off side more frequently. This means that when the ball is straight you not as well aligned and have to get your shoulder round more. You also risk being bowled as you are not covering your stumps.

 Middle stump  

Players who hit the ball well in the 'v' and especially through mid on prefer this guard. I would advise you to take middle for the following reasons:

• You minimise the chances of being bowled.
• You are well aligned and therefore minimise the chances of playing around your front pad.
• It is easier to make the decision to leave the ball outside off stump because as soon as you turn your shoulder to play on the off side this is a good cue for you to let you know you can leave the ball.

Middle and leg (or 2 Legs)  

This guard is a compromise of the previous two. You are at less risk of being bowled compared to the leg stump guard, but more risk than the middle stump guard. However, you are at more risk of LBW and your eye line may be outside off stump, causing you to play at balls you can leave. 

Your guard is a personal choice (remember it's only a matter of inches). Know the benefits and risks and you can select one that best suits your game.