What type of brush to use with polyurethane?
Back in the day, it was natural bristle for oil finishes, nylon bristle for water based. Polyester bristle came along touting good for any paint, oil or water based.

Just as a mention, the foam brushes do a very nice job of laying on a finish, but I would only use them for small jobs, the core wears through the foam under long time use. I do not use them by choice, but I have used them here and there over the years and they are my second choice.

In my experience nylon is crap for oil based paints. A good grade, mid priced brush is what I use on water based paints when finish is important. Here I will not hesitate to used a mid priced polyester brush too.

I use a mid priced natural bristle or polyester brush for my finer work, and the results I have seen under close scrutiny with a bright light looks smooth and free of brush marks.