on a hot day,which would stay cooler longer a sheet of aluminum (specific heat 920 J/kg) or copper (specific heat (380 J/kg K)? Why?
If you're already installing pex in the rest of your house, then there's no reason to change. I would be interested to see the research that shows copper will last 100 years. That would have to be with absolutely pure water running through it, or no water running through it. In our area, where the lime content is quite high, I repair at least a dozen in-wall copper line leaks, (usually a pin hole that corroded thru), each year. Most of these houses are less than 40 years old. I'd say Pex is the wave of the future.
pex is faster, easier, and cheaper. it will last just as long as copper, if not longer. the best advantage of pex is if it is used in a home run system there are only two places it could possibly ever leak from: the stop at the fixture or at the manifold which will likely be in you mechanical area near the water heater.
PEX meets code, is used extensively by production home builders and is growing in acceptance in the housing industry. Having said that, I would use copper. I know as a matter of fact copper will last a hundred years, probably much longer but I don't know that as a fact, if not subjected to radically altered water or physical abuse. PEX is usually warranted for 30 years but I have been bitten by several warranties in the past. I just built a 3400 SF home myself and sweated copper for all water supply lines.
Based on the info you privide, the answer is the aluminum since more heat is required to raise its temperature for the same mass.
