The things that are touched by you in the normal operation of things are better served by the copper. The plastic can get brittle. If it isn't getting stressed, then it is usually okay, but if it is someplace that can get bumped or used as a handle, or moved around on a regular basis, you would be much better served (in my opinion) with copper or brass. That includes the connection in the wall that originates the pipe coming out of the wall for the showerhead and tub/diverter spout.
My best suggestion is to buy some fittings, solder, tubing cutter, soldering torch, cleaning brush for inside and outside, and practice a little bit in the garage or outside. It isn't really all that hard. Otherwise, it shouldn't cost much more than the minimum for a plumber to do it for you. Do you have any friends that you might tap to do this for you? Really, beg, borrow, or buy the tools you need and try it on some scraps that you can throw away until you are confident. The next time you need to do it, will be much easier and less traumatic.
Now, if you live in MA, you aren't supposed to touch your own plumbing (although I'm sure many people do); a plumber is required.