Both survive outside all year. They just ask different things of you.
Poly lumber
Poly is high-density polyethylene — the recycled plastic behind most of the seating and tables on this site. It does not absorb water, so it will not rot, splinter, or crack in a freeze. Color runs through the board rather than sitting on top, so a scratch does not flash raw wood underneath.
Upkeep is a hose. No sanding, no sealing, no repainting. Weight is the trade-off: a poly Adirondack chair is heavier than the same chair in cedar, and it will not develop a patina.
Western red cedar
Cedar ages honestly — honey-brown when new, silver-grey after a season or two of weather. It is naturally rot- and insect-resistant, so bare cedar lasts years outside without a finish. A clear sealer keeps the warmer tone; skip the sealer and you get the grey.
Cedar is lighter to move and to hang (a real advantage for porch swings) and softer under bare legs than poly. The trade-off is upkeep: check fasteners each spring and reseal every year or two if you want to keep the color.
Which to buy
- Zero upkeep — poly. Leave it out all winter.
- Traditional look in a lighter piece — cedar.
- Porch swings — cedar is easier to hang; poly never needs a reseal.