some thoughts.

I think you have to find out how deep you have to bury this thing to not hit the temp swings you have out there.
whatever you use, you want to minimize the expansion and contractions that might cause.

I think there are many plastics that would last a long time due to it being buried and not exposed to UV, which typically makes plastic brittle.

If concrete is that much cheaper, have you considered what they coat pools with? you would have to do the research on why they have to be refinished, but I have to believe they hold water without imparting chemicals into it.

how are you going to keep the water from stagnating or growing stuff? or is this just a pass through tank for your water supply?