3M is considered the best for wrapping, but you can get away with cheaper stuff. The difference is in the curves, the top stuff will follow curves with no wrinkles, no difficulty and it will maintain the curves. As you go down the grades it gets harder to follow the curves, working with it gets more difficult, and it just gets horrible. As a
guide, a roof can be done in cheaper stuff, but if I was attempting a wing mirror I'd aim for 3M.
My humble opinion is to aim for the middle ground. MDP supplies (earlier in this thread) do three main types of vinyl. Cast vinyl, which is top notch and really easy to work with. Polymeric calendared, which I believe is the one of the best calendared vinyl, and monomeric, which is the bottom calendared. My choice would be the polymeric, although MDP's cast vinyl isn't that expensive compared to other places I've seen.