As far as I know, there is not an M14 made with a metal body, as the real ones don't have it. And neither ICS or CA make an M14.
I would absolutley 100% recommend a Marui M14 OD. I've had mine for a month or two now, and friggin love it. You'll be amazed at the range and accuracy stock, even if the fps is only around 300 (haven't chronoed mine yet, but they're estimated at roughly 300 fps stock). It is also extremely solid, well built, and has a great feel for it. Word of warning though, it is heavier and longer than most AEGs. I was surprised that after playing for a while with it, my arms started getting a bit tired. Nothing serious, but now when I use my M4 it feels really light in my hands.
As far as the metal vs plastic debate; metal is more realistic, plastic is nice if you're gonna be running around all day. Realism is fine and dandy, but come on, we're shooting little plastic bbs at each other. My M4 is a plastic body, and I don't mind it at all. I very well may get a metal body for it, but I'm not planning on it right now.