It really all depends on what you're trying to accomplish. Some people like to use one, some people like to use the other. If you are needing to be able to reshape it easily, then chicken wire would probably work best for you. On the other hand, if you are trying to get really nice smooth, straight lines, then forming the foam in the desired shape will enable you to really fine tune your shape.
I don't know what you are making out of this enclosure. Is this for an amp rack or a sub enclosure or what? If you're making a sub enclosure, you may want to consider making a basic frame or mold out of either mdf or fiberglass, whichever you want, and then using mdf rings and some sort of stretchy material like fleece. If you're going for an amp rack, then you may want to build a box out of mdf with the top side open, then again using fleece or something similar to form the shape.
