We just bought a 42" crate for Jess. I picked one with a plastic tray - figured easier to clean? Also did borrow an "old" crate from a friend that had one with a metal tray and we found it used to flex under Jess' weight and as we have a babycom (AKA labracom) in the kitchen at night, she woke me up (not the OH!) every time she moved.
IMHO, a plastic one is better but someone else may suggest metal being best?! :?
Plastic trays can crack and have very jagged edges when they do. Once a crack starts you tend to find the dog 'worries' at it. Metal ones are damn heavy... thats their drawback... and usually more expensive, but they don't crack break or tear....
I have to say we plumped for a metal tray in the end due to the fact that our old girl had an old plastic bed that she managed to chew the edges clean off of 8O I had visions of the great escape if the new pup managed to chew that much plastic off the tray