Different systems offer different pro’s and Con’s. and loads of factors need to be considered on each installation.
UV resistant Poly systems tend to be lower cost but have some life limitation ( the length of the guarantee is usually an good indication, also look for the small print in the guarantee)
Copper tube/flat plate collectors , usually are quite efficient and generally have a primary and secondary circuit with a titanium or stainless steel collector, meaning immunity from the chemicals and salts etc, as indicated above, these can also be integrated into a DHW for central heating system to act as a preheater or supply DHW demands when surplus is available.
Vacuum collectors can be very efficient, again make type is important,
Heatpumps are acost effective solution ( install cost)., reckon on about 3500?/4000? for an 8X4x1.5 pool with running cost can be as little as 450? P.A.
Heatpumps can be integrated with a Solar heating system to provide a very overall effective solution, providing good heating even when there is little or no sunshine (Make sure your power supply can handle the start up current on a heatpump system!!!!}
At the end of the day you get what you pay for, key points is to ask for references, discuss with other customers, and go look at installations,