I'm leaning towards copper, depending on the response to the above.
PTEG:
1. Mine has been in ages, i've got no issues with it...
2. Yes max size is 16/12
3. Copper is complicated as the sizes you need for water cooling a PC are not really standard sizes for plumbing. So you either get a special size, or pay a little over the odds vs. a hardware store to get specific water cooling stuff. (its the same)
"Common sizes are 15 mm and 22 mm. Larger sizes are 28 mm, 35 mm, 42 mm and 54 mm"
Copper:
Main issues are:
1. Bending is tricky and can be less adaptive to some bends
2. Size is not a standard size i.e. the usual pipe for plumbing at home is not in the right size typically
3. Fittings are not necessarily guaranteed, if you get metric fittings (mm not inches) should be good if the OD is right but again nobody will guarantee you that
That said I recently saw some chrome plated copper tube and it looks amazing. Very pricey though...
People tend to disagree a lot with the material but I can honestly say PETG has been great for me. Up to you mate, Greebo might add some more or disagree as he is currently plumbing in a million rads to cool his new kit.
Hope it helps.