Looking at the area the steps are installed on and the algae around them, made me think the climate is better for the acrylic. If you are a painting contractor, you understand the why of that. You would also get why the oil might be less favorable. As for brand, well, do you have a professional preference? Go with what you know.
Beth
p.s. stringers are a pin in the butt. if you have to replace a step there is a good likelihood they crack and break if they are older......