Either can be great so long as the rack was built well. I prefer pvc now due to weight and it not being susceptable to moisture getting into it, but I also have melamine racks that were built back somewhere around 1990 and are still good units. I recently built another 41qt rack out of melamine because I still had some sheets of it that I could build it out of (I probably bought those sheets in '94), but if I'm having to buy materials for another rack or cage, I'll be buying the pvc sheets.