cathode tubes tend to be more sensitive, and the power packs (transformers) deliver very high voltage (AC I think). They have to be carefully sealed if used on the outside of your car, and regardless of wether you mount them inside or out, the wire that connects the powersupply to the tube cant be extended (you may be able to get some extension cables made for the kit, but even then there is a max distance). Talk to Phil at oznium.com. I purchased two 15" neon tubes from him and he was real good about answering my questions about neon vs cathode.
The pic below is neon: