Honestly, wood needs more consistent maintenance than metal, but “metal” is a broad category. I’ll break it down by material because that changes the answer a lot.
For wooden outdoor benches, the biggest job is protecting the surface from moisture and UV damage. A gentle wash with soapy water and a soft brush every few months will remove dirt, pollen, and bird droppings. Depending on the wood, you’ll also want to apply a sealer, wood oil, or stain about once a year. Teak and cedar are more forgiving, and they can be left to weather into that silver-gray look if you like it, but even they benefit from an occasional cleaning. Softer woods like pine need more attention because once the finish wears thin, water gets in and you start seeing cracks, warping, or mildew. You may also need to lightly sand rough spots before reapplying finish. It isn’t difficult work, but it is a regular weekend task.
Metal benches are generally easier, but it depends heavily on the type of metal. Powder-coated steel or aluminum mostly needs a wipe-down with mild soap and water a few times a year. The main thing is to check for scratches or chips in the coating. If bare metal is exposed, moisture can cause rust, especially with steel or wrought iron. For those, sand the rust spot, apply a rust-inhibiting primer, and touch up the paint. Wrought iron is the most demanding metal because it can rust from the inside out if the coating gets damaged. Aluminum won’t rust, but it can oxidize and look dull, so an occasional wash and maybe a coat of automotive wax keeps it looking fresh. Stainless steel is low-maintenance, though it can develop tea staining near salt water, so rinse it more often if you live near the coast.
In practice, plan on a deeper maintenance session once or twice a year for wood, while metal benches often just need seasonal cleaning and a quick rust check. If you love the warm look of wood and don’t mind the annual sealing or oiling, go for it. If you want the least upkeep, choose powder-coated aluminum or high-quality stainless steel, and avoid unprotected cast iron or cheap steel.