Gary,

It sounds like you agree with Nick, that copper pipe is a waste of
money for the pool HX because without pumped (presumably turbulent)
flow on both sides, at least one water film is going to dominate the
series resistance. I certainly agree that once you go with plastic
tubing, it's worthwhile to err on the side of too much pipe. It would
be very nice if the plastic tube HX got a U anywhere near 125
BUT/hr-ft^2-F, it'll warm the hot tub up in a hurry!

I don't have any sense of how big a deal this oxygen diffusion is.
Supposedly the PEX-Al-PEX tubing is better than just PEX specifically
because the Al acts as an oxygen barrier. I guess the dissolved oxygen
is supposed to eat your metal bits over time. If I use PE tubing for
the pool HX, I assume I'll be pumping oxygen into the cistern water.

The oxygen issue seems slightly different that the open vs closed
system issue. Supposedly open systems that expose the water to air
require more maintenance. My assumption is that open systems
accumulate both oxygen but also organics and bacteria and whatnot,
which then all has to be killed and strained from the system to prevent
accumulation in the pipes and so forth.

For the DHW HX, it seems to me that the dominating issue is safety, and
copper tubing seems safer than plastic tubing: Less likely to leak or
crack, less likely to blow out (stronger), less likely to support some
nasty thing growing. Probably more likely to leach poisonous heavy
metals into the stagnating hot water, but that's why we don't make
coffee from the hot water tap.