It's important to make one thing very clear when discussing piping materials for a solar thermal installation: copper or stainless steel are currently the only sensible choices.
It may be tempting to try to use a flexible material like rubber or Pex, but it won't work: there's a very good chance you'll have to replace all your piping within the first year of operation because it simply cannot withstand the constantly high operating temperatures of the system.
The one exception to that is using Pex piping for the cold water return to the collectors, where it won't be under such a lot of heat stress. But it's still important to terminate the Pex at least 10 feet (3 meters) away from the equipment and, if the piping is laid outside, to bury it.
Solar thermal installations require pipes that retain their integrity in a temperature range from -30°F (-35°C) or lower to 300°F (149°C). Traditionally, only copper can handle this, though modern technology has produced a stainless steel alternative.
Copper pipes come in a variety of grades, categorized by their rigidity and wall thickness. Thankfully, they're all the same size on the outside so there's no problem with calculating space requirements, though you'll need to allow an extra 1/8 inch on top of the nominal designation.
The three types of copper piping – K, L and M – designate the pipe's wall thickness, running from thickest to thinnest, and the material is either annealed and soft (which can be bent) or drawn and hard (which is very rigid).
For solar thermal installations, it's usually best to install drawn L piping everywhere except underground, where annealed K is a good option – though you should still discuss your individual needs with a professional.
The stainless steel alternative to copper piping is relatively new to the market. It's been available long enough to prove itself a viable option, with one major advantage, two notable drawbacks and one significant limitation. The piping is made of corrugated stainless steel and usually comes in a long roll with the supply and return lines encased in high-temperature insulation; a sensor wire is often included, sandwiched into the insulation.
The advantage of stainless steel piping is that it's extremely flexible and easy to install, which reduces labor costs significantly. On the down-side, that flexibility comes at the price of corrugation, which increases the surface area inside the pipes. This, in turn, increases friction head (resistance to flow) which means you may need a bigger, more powerful pump to move the water around your system. That consumes more electricity and raises your operating costs.
The other disadvantage at the moment is the cost: stainless steel piping is much more expensive than the copper counterpart. The price is normally offset to some extent by the reduced labor costs (and the reduced ecological impact of mining less copper) but it's still a consideration.
If you're installing a drainback system, you'll hit the limitation of stainless steel piping. The solar fluid in your system will accumulate in pockets created by the dips in