AAAAHHHHHHHHH!!!!!!! I hate this topic. You will hear so many different posts. Here, check out this link. I guess it is just personal preference. Urethane is more durable, but heavier. You shouldn't really need something very durable. If you are going to be running into things, then don't get a kit. Know what I mean? To each his own. Hope this helps you out.
https://celicahobby.com/feature/bodykits_faq.html