I've used both kinds.
The plastic one is cheap and works well for most things. It's also really easy to build with locally available parts. If do you clog it with a stray bit of yeast or hop, it has to be back flushed though. Also, only minimal pressures are usable due both to the construction of the thing and to the carboy itself. After many uses, I wanted something more rigid as it was fragile and a PITA to clean thoroughly.
The metal ones are easier to clean in place, and they enable greater pressures, which can be useful if you are fermenting in a keg.
honestly, either is good.