Is fiberglass well water tank better?

Hi, We have a Well-x-trol WX-251 tank. It has a lot of rust due to condensation (please see pictures) http://i48.photobucket.com/albums/f245/Novice973/Water%20Tank/20150429_061414_zpsig7phbyk.jpg Does it look like it is time to replace the tank? The tank was built in 1997 and installed in 1998. Should we go with a fiberglass tank instead? I was told the fiberglass does not work as well as the Well-x-trol. Our water is very acidic. We are constantly replacing the copper pipe from the well to the PH balancer due to pin hole leaks. My husband wants to use PEX. I am concern of the chemicals that are leaked into the water. We will be adding a reverse osmosis system. Will that take care of the chemicals from the PEX? Thanks!