Definitely go for metal unless you're going to make them yourself. We've been here ten years now, and most of the wooden gates etc that I made when we first moved in here now need repair or replacement, whereas the galvanised steel hurdles we bought are still as good as new.
Something to beware of, you definitely want galvanised steel rather than painted. However, the more expensive ones will have been welded up and THEN galvanised as a complete unit, whereas cheaper ones are made from pre-galvanised tube welded together. However, these tend to rust at the welds after a while, and are a false economy IMHO.
Hope that helps!