Sure heres a thread with some loadouts and explanations on how to use nob bikers.
http://www.dakkadakka.com/dakkaforum/posts/list/330905.page#2181822
For your second ? its fairly simple. Are you going a full biker horde/speedfreaks build? If so they are both rather important. But if it comes down to having to pick one or the other, the Nob Bikers win hands down. They are known as a deathstar unit, and can rip up anything you send them at.
If you want more info on how to use biker boyz, do a search here in the tactics section on them, youll find a few threads with good discussion on the ways to use them. Ive seen people use them as
cc units, or shooty units, thats the way you use them on the 2 basic levels.