Wooden picnic tables certainly look cheaper initially, but the cost of owning them tells a different story. A wooden picnic table or bench will steadily deteriorate and in no more than 5-7 years will need replacing, even if you shell out for annual maintenance. Labour and materials don't come cheap. And preservative treatments are no longer as effective as they were, because the chrome, copper arnsenate salt used in them was toxic and has been banned. Recycled plastic picnic tables are favoured by PTA's and local authorities because they are happy to make a long term investment in a no maintenance product that has no splinters, is weatherproof, vandal resistant, non toxic and will last a lifetime.
See WRAP Cost Comparison Study