Jdutton = what kind of painting or finishing have you done in the past ?
what kind of brush do you normally use ?
what part of the world do you live in ?
when you say: Polyurethane; is it oil or water based ?
inside or outside project ? and the project itself.
painting and finishing is a learned skill.
what really comes heavily into play is the atmospheric conditions (temperature & humidity) of your area, what finish you use, how you thin it, how much you put on your brush, how thick of a film you apply in one coat, yada yada yada.
I have brushes that cost $25.00 and more. I also have brushes from Harbor Freight.
on some projects, I can achieve the same exact finish using both.
so just suggesting one brush, or type of brush, is not an adequate answer to your question.
practice, practice and more practice with different finishes under different conditions
will make you a better finisher.
Edit: when you have time, you may find this article helpful if you intend to
be doing much painting and finishing in the future.