Honestly, the short answer is that a quality resin park bench will usually outlast a treated wood bench by a wide margin. A well-built resin bench made from recycled HDPE or poly lumber often lasts 25 to 50 years with almost no maintenance. A pressure-treated wood bench, even when maintained, typically lasts around 10 to 15 years before cracking, warping, or rot becomes a real problem.
The reason is simple. Treated wood is still wood. Pressure treatment helps it resist rot and insects for a while, but water can still get into the end grain, screw holes, and surface cracks. Over time, sun and rain cause it to expand and contract, which leads to splits, warping, and splinters. You also have to clean it and recoat it with stain or sealer every year or two. If you skip that, the lifespan drops fast.
Resin park benches are different. They are usually made from dense recycled plastic, so they do not absorb water, they do not rot, and insects do not bother them. There is no sealing, staining, or painting required. Good resin benches have UV-stable color all the way through, so they may fade slightly after many years but they stay structurally sound. In freezing winters, hot summers, or salty coastal air, resin handles the weather better than wood.
There are some trade-offs. Resin benches usually cost more upfront, and they are heavy. Treated wood looks more natural and costs less initially. But when you factor in cleaners, sealers, replacement boards, and labor, wood often ends up being more work and more money over 20 years.
So in a direct comparison: a treated wood park bench will typically last 10 to 15 years, maybe 20 with excellent care. A resin park bench will typically last 25 to 50 years, and some carry lifetime warranties. If you want the longest-lasting, lowest-maintenance option, resin is the better long-term choice.