ERD50
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
If I have a choice for pipe inside a wall (new construction or an addition), it would be PEX. But, my house is all copper and the pipe is in good shape so I generally fix what I've got using copper.
I would do that also. When I redid our kids shower/bath, I had to jog each of the supply lines a bit to get them to line up with the new valve which was wider than the old (probably because of the pressure balancing anti-scald feature, which BTW is worth it's weight in gold!).
So that involves a couple 45 degree fitting on each side, plus everything else to get everything together. I had over 20 joints to sweat in that wall. I was able to do most of them as an "assembly" outside the wall, and then just sweat a few in the wall, that helped. But it still made me nervous. I had one very slow leak, amazingly on a lower pressure side (the pipe going to the shower head). Fixed that, kept the wall open for two weeks, just to make sure nothing developed from a few pressure cycles (well, that was my excuse anyway....)
PEX would have made that much easier.
-ERD50