I just replaced all the plumbing under my house with some new stuff called Pex. Prior to that I knew next to nothing about plumbing. I had some trouble getting the rings that secure it to the fittings tight enough because I didn't have the best tool for clamping them. I was finally able to borrow the correct tool and it wasn't easy but it worked out fine once I got them tight. I would think that iron wouldn't be the best because it would rust and it would be harder to work with. There are different kinds of plastic that you use glue on. There are also other kinds of fittings that you can slip on with the Pex but the kind that are rated for between walls and under the house are expensive. If I had it to do over again I would still use Pex but if I could afford it I would use those fittings or just make sure I had the correct tool for clamping the rings. It was about two feet long and you clamp it over the rings and squeeze them tight. It was a little difficult in tight spaces but I was able to make it work. There is a smaller tool that you use with a pair of Vice Grips but that didn't always get the rings as tight as they needed to be. I think Pex is the way to go because it's easier to work with than plastic (actually the Pex looks like a type of plastic but it's a little more flexible than that hard plastic that you have to glue) or iron and it has enough give in it so that if the pipes freeze they might be less likely to burst.
Pex is the future of plumbing. My plumbers have been using it for a few years and most will not go back to copper for water feeds. Like you said, all you need is the clamping tool. With copper, you need a torch, flux, solder and a truck full of fittings. PVC plastic has been used for drains and vents in the US for many, many years and has replaced iron pipe.
Definitely not iron. Iron rusts and will detererate. Type L copper is good. Plastic pipe, like Pex is becoming popular. It's quick and inexpensive (well at least the pipe is... fittings are not). You need to rent or borrow crimping tools to install Pex pipe.
What application are you referring to water distribution or drainage? If it is water pipe plastic wether it be pec or cpvc is the best all around. Drainage pvc is the way to go unless your worried about noise?