What is the correct type of material, 304L or 316L stainless steel tubing, for transport argon gas? Is it required to be an electro-polished finished?
The mechanical and manufacturing properties of 304 and 316 are not greatly different and both have good weld-ability (for stainless). The main difference is that 316 has a bit better corrosion resistance and is genreally rated as performing better in more corrosive environments (salt spray etc).
Having said that industrial argon cylinders are generally non-stainless so unless there are other considerations either should be fine in principal.
However, as noted in comments there are usually strict regulations associated with the design and manufacture of pressure vessels and anything used with high pressures may well need to be certified. This is greatly simplified for piping is you specify a grade which is already certified for its intended use along with the appropriate fittings and connectors.