I've heard both when it comes to sanitation - plastic vs wood.
I watched a tv program where they went into some home kitchens and tested for bacteria. They used an indicator chemical that turned red when bacteria were present. It was everywhere - on the table, the rag, the sink, the counters, the cutting board, the fridge handle, inside the fridge, etc. They found wood boards tended to have less bacteria in the average home.
Wood has natural ingredients that retard bacterial growth, plastic does not. Plastic can be cleaned with harsher chemicals, but will still support the bacteria after cleaning. Either can be kept clean. Its your routine that counts.
You can contaminate a clean board with a slightly dirty rag or unwashed vege's. or by setting it on a "dirty" surface. Any surface can look clean and harbor bacteria.
I dont see many wood boards in the restaurant kitchens I visit, but I do see some. I don't know whether the health departments have rules against them. I would expect them to be illegal if they are so unsanitary.