I have traditionally used the "old-style" right turn signal (left hand up bent at the elbow). I find that if cars are following me (even if I'm on the shoulder or bike lane to the right of traffic) cars will simply slow down because they don't know what it means. It doesn't matter to me, as long as I don't get hit.

The other disadvantage of using your right arm is that if a car is at the right angle behind or beside you, they won't see your signal. Your left arm is always in front of traffic (assuming your country drives on the right side of the road), so it is always the most visible "device" for signaling.