PEX is easier to install, therefore saving you money. Some people are
still terrified of PEX, due to an issue with an entirely different kind
of pipe (poly) in the 80's. It's actually more resistant to bursting
from freeze than copper.
Copper causes cancer, according to Effi. Other than that, I'm not aware
of any downside to copper, other than it bursts easily when frozen.
I'd guess in another 10 years or so, copper will be phased out
completely in the mainstream.
Having said that, were I building a new house, I'd choose copper. No
real reason though, other than that's what was in my parents house, and
has been in every house I've ever owned. I don't need any new
inventions from you young wippersnappers.