I would recommend the type that clamp on to the top of the mirror housing. I have a pair of the Towsure Rock Steady mirrors that clamp on to the tom of the mirror and also have a pair of rubber straps that stretch round the back of the mirror and hook on to the bottom edge. Milenco mirrors also seem to be popular.
I would not recommend the type that sticks on to the surface of your wing mirror with a suction cup. We had these at first and they kept falling off.