Both EPDM and TPO are rubber compounds, but TPO appears and feels more like a plastic sheet.  TPO is a thinner material, so care must be taken when replacing an EPDM, or traditional rubber roof, with a TPO roof. In this case, the plywood base may need to be raised up so that good drainage of water along the edges of the roof is maintained.

TPO is generally less expensive than EPDM and can be easier to work with as well