well it all depends on the pricing of gas to electric in your area. Electric heaters could prove to be 98% efficient as opposed to gas being 85%. Also the way it is set up plays a big portion in the efficiency department as well.
Your talking about PEX Tubing I believe. Theres a type called PEX-AL-PEX which comes with an aluminum lining. It is specifically designed for heating applications. Take a look at this thing. http://www.canarsee.com/pex/pipe/1-2-in-x-300-ft-pex-al-pex-tubing?___SID=U