My personal experience is the opposite of Tury's.
Plastic would pinch and be uncomfortable, would be hard to clean and didn't ventilate well (the cb-x series anyway). I had to take it off at least once a day for thorough washing because it turned into a horrible, dank swamp in there. If you have a foreskin, as I do, it can get worse, and I had an issue with it being constantly swollen and irritated while caged. They also tend to be incredibly annoying to put on without some sort of lubing up first.
My current cage is metal (this one
http://g01.a.alicdn.com/kf/HTB18E04KFXX ... ge-Hot.jpg) and I've had no pinching issues (it's only two parts, and none of those parts have any large, flat surfaces as points of contact for skin or hair to get trapped between), less slippage issues, it is more comfortable, I don't get the sweat/hygiene issue and it is very easy to clean while wearing.
Yes, it is a bit heavier, but it does not weigh THAT much, and I've had fewer issues with it than my other cages.
Both the plastic cages I purchased broke less than a month of use later. One of them literally split down the center seam where both halves of the cage meet, because of morning wood. And they were three times the price, each, the cage I have now.
Honestly, odds are, unless you can go to a trade-show or a shop (who knows if they exist where you are!) to get either a custom fit of personalised advice based on your anatomy, you're going to have to try out a bunch of different models.