Bruce, if at all possible get the brass one to replace yours. The cartridge I get comes with a special tool to assist in the removal. The current one may be very difficult to remove if you do not use the tool to twist back and forth to release the "o" ring from it's frozen state.
Even after removing keeper pulling the stem may be impossible to remove, even disentigrate if you dont first twist. If this happens it is even more difficult to remove w/out something to grab.