Honestly, the biggest difference in upkeep comes down to moisture and coating care. A wooden outdoor bench needs seasonal attention. I usually wash it with mild soap and water, let it dry, then apply a fresh coat of wood sealer, teak oil, or exterior stain once a year. That keeps water from soaking in and helps prevent cracking, warping, mildew, and UV graying. If you skip it, the wood can develop splinters, dark spots, or rot around the joints. Hardwoods like teak and eucalyptus are more forgiving, but they still benefit from cleaning and occasional oiling if you want to keep the original color.
Metal benches split into two camps. Powder-coated steel or wrought iron benches require you to watch for scratches and chips in the paint. When bare metal is exposed, moisture gets in and rust starts. I always keep touch-up paint on hand and spot-treat any chip before rain arrives. Aluminum and stainless steel benches are much lower maintenance—usually just soapy water and a rinse a few times per season. They do not rust like iron or steel, although aluminum can oxidize slightly and stainless steel can get water spots if left dirty.
So in short: wood asks for more regular sealing and refinishing, while metal asks for less routine work but rewards quick rust prevention and scratch touch-ups. If you want the lowest upkeep, choose powder-coated aluminum or stainless steel. If you love the natural grain and do not mind a yearly afternoon of care, wood is still a great choice.