I have had good expereience with SKS mudguards. Ultimately, the question is how much non-riding travel is the bike likely to do. If it is to be boxed and sent via airline, or even on a train, or in the back of a vehicle, then I would strong suggest plastic. It took a lot to break my SKS guards (and it was while I was riding, atlhough the story is a bit complicated to go into).
It's also amazing how much close-quarters manoeuvring I have ended up doing with a loaded touring bike, often in crowded situations. It's then the foregivingness of plastic (or in the SKS case, plastic-aluminium sandwich) comes to the fore.
If the bike is only for local touring, or you are fine with carefully packing, then metal guards might be OK.