What material to use for a prop sword?
Wood. Adam Savage (whose non mythbuster stuff I love to bits) has a tendancy of making them - see his hellboy sword. He used thin plywood for a base and shaped it with bondo, covering it with aluminum ducting tape. He has another video where he shows process of making a shiny not weathered 'metal' sword from a solid wood sword - the process where he covers the sword in tape starts here, but the whole video is awesome.

There's actually a precedent for this with solid boken, and many modern sword shaping techniques might work with wooden swords.