Go plastic, metal ones over rated. Basically the seals eventually go, this exposes the bearing, they rust and impeller disbalances. A plastic one will grind down and cause no damage. Now imagine a metal impeller disbalanced this will grind a bore away, metal to metal contact.
Plus the new plastic pumps were revised a long time ago. Still advise you change the pump though with every cambelt change though, mine wasn't done at 60k by previous owner and at 80k the seals had already started to leak.