For me it depends. I prefer the plastic crates in some situations, metal in others.
When it comes to transportation, assuming you're not using a metal dog-box, the plastic crate is the best. I can carry two halves in my car, snap it together when I get to the field, and if the dog gets the inside muddy from working, it's easy enough to clean on the spot, or throw some paper between the halves when I take it apart, and hose it out at home.
Wire crates I find a pain in the arse to set up and take down.
That being said, plastic crates are not necessarily designed for long-term regular crating, and I've had the plastic show stress lines from constant use. Also once they start to break, there's no real fix once the plastic cracks. Wire cages can be jury-rigged in a pinch.
Generally I only use metal crates in the house with dogs I know won't flip them, rip up the pans, or mess and splash it everywhere. I have more plastic crates than metal ones, because I find them more usesful overal; especially since my dogs often travel, and not needing to rush-order get a GSD-sized shipping crate at the last minute is always nice.