After seeing the same question posted several times a year for the last 6 years, Some would think folks would actually do a search on the topic. When you been here a while, and actually contributed something other than trolling the pot, then we can talk. Some of us not only spend time on here reading, but try to read through real issues to help folks. Some even actually give their time to help others in the field. Others come for a few months, then move along. If you want to debate me about this, please PM me so I can privately tell you to go F yourself. Back on topic: Each has their own preference. Some manufacturers spec poly, others say either is fine. I called Simpson back when, before they merged, and the informative tech there said steel was perfectly acceptable. I have used both, and jumped on the poly kick a couple seasons ago, but will say, the poly does clean ok, but seems to me the steel cleans better. Maybe if they made the poly with more bristles like a steel brush has for more cleaning area. This year it was very damp out, and the crap inside was moist with dampness, the poly just does not have the brush to pipe surface coverage that the steel has. Spinning and brushing does help, but just seems the lesser surface contact the poly gives, does not get as much as the steel does. Of course this is in my set up. Everyone's mileage may vary.