One might measure "power" as it pertains to guns in how much damage it could do. Now, that teenager probably didn't realize it but he could very well have been correct. Though a .357 mag can deliver more ftlbs of force upon a target, it is usually restricted in the capacity of the weapon it is fired from. Now, one of the most common Glock 9mm pistols is the 17. The glock 17 usually holds 17 rounds in a mag but can be loaded with those crazy 32 rounders. The typical .357 revolver or semi holds 6-8 rounds. Assuming that you can get a kill shot from every single bullet in a mag or cylinder the 9mm glock has more "killing power/potential." Though the .357 can really kill the crap out of you and can probably do it more efficiently than a glock, there is a reason why hardly any police force carries a .357 wheel gun anymore. I actually know of none but I don't doubt that it is possible.
I am not trying to say that the 9mm cartridge is more powerful than the .357 cartridge. What I am saying is that the 9mm glock has potential to do more damage than a weapon chambered in .357.