Definatly wood! We've had both and the wooden one will "age" better and not look so scabby in a couple of years. It seems to be stronger too. I bought my wooden one from a garden centre in Carlow, name escapes me at the moment but if you want it, I'll dig around and find it for you. He was good because he allowed us to substitute the plastic seats for wooden ones and we added a couple of bits. Cost about Eur 450 in the end for a two swing and slide combo with a rope and extra swing but he does smaller combos.
Let me know if you want his details, like I said, I'd have to dig around for it. Think he delivers too.