but im stuck as to what material will give best grip,
polyurethane, elastic rubber etc,
Have a look at this article, it might help you

http://www.thombert.com/white_papers/Po ... rTires.pdf

To be able to compare you have to know the durometer of the wheels you're comparing, as polyurethane hardness can vary over a wide range, but most probably being heavy duty wheels rated at 800kg, they will be of the hard type....

Hope this helps