The pelham is stronger due to the curb chain and the effect of the lower ring, it is almost the effect of the curb it self!
Then you have several tipes of pelhams, some are shorter (lighter) some are longer (stronger bit), some are articulated some are one hole piece, some are of rubber some of metal, all have different effects and responses!
I believe pelham is stronger then a sniffle but less strong then a double bridle!
Hope it helps
Pelham is longer and has two reins
The Pelham is considered as a strong bit, close to a double bridle (you can use it with one or two pairs of reins) it can have different effects:
-on the snaffle rein, it acts like normal bit and is not mean, it only has a lower effect,
-on the bridle rein, it has a raising effect, supported by the curb chain (no need to tight it a lot)
You can also use it with bit converter to limit the effects on the horse's mouth.
It is a strong bit that needs to be used with a light hand and experimented rider.
I hope i could help you :)