Sani- The argentines seem to have a preference for gags. In their high-goal, you will see gags predominantly. In the US high-goal, however, pelhams seem to be much more common. Not exactly sure why. I asked someone far more knowledgeable than me. Her answer was that gags kept the horse more on their front end with their heads down in a running and flowing motion where the pelhams tended to keep the horse more in a checked up position on their hind ends. Not sure if this is right. Maybe some others with more knowledge could weigh in.
I believe that because of the leverage effect of the pelham it is much easier to stop with less power of your arms than with a gag. More longer the pelham more dangerous it is. (I saw broken jaws with some long pelhams)
The gag also has the effect, that's why the name in spanisch also is "levantador" = lifter, that it get the head of the horse higher, to lift the front legs when you stop, what's better for the tendons.
After all I believe there is always the right bit for each horse and the riding method of each rider, I personally wouldn't say one bit is better than another one.