I own both. Each has their advantages and disadvantages.
My M92 has a metal slide. It have a really great sound when you rack the slide. It shoots well with a decent kick (not very accurate though.) It's a heavy gun with a realistic weight(about 1kg.) It's not as gas efficient compared to a plastic slide. The metal isn't that great. WE tend to have crap paint jobs so it wears exposing cast metal.
My TM M&P 9 has a plastic slide. It actually looks more realistic but the lack of weight gives it away as being plastic. It still has a an ok kick. The slide racking sound isn't that good compared to metal.
ABS for a pistol to play with since it won't let you down in the cold and will perform really well. Metal slide for a holster filler, the noise of racking a nice metal slide on a well put together pistol is sublime.