I have a bench from Plow and Hearth. Very nice plastic with uv protection but it only hold 300 pounds which usually isn't =to 2 people. so it is good mostly for kids.
I also have an old fashioned park bench (metal frame with wooden slats) which I have had for 20 years or so. still comfortable and if the slats break you can replace them. That is what I would go with.
Another choice I would probably try to make work is the adirondack chairs in plastic. VERY comfortable and light enough to move and put away in the winter.