T409 is widely used in the automotive industry as it has good corrosion resistance but is cheaper than T304.   T409 will form surface corrosion but doesn't compromise the structure of the material.  Surface rust can be cleaned off relatively easily with wire wool.  It is easier to manipulate than T304 and as easy to weld as mild steel.  T409 will polish but not to as high a standard as 304 or 316.

The standard finish is fine in situations where the tube is not being used for decorative purposes.  Standard finish material can be bright polished to a mirror like finish but generally it will come descaled and looks acceptable in most circumstances where functionality is more important than looks.

Thin wall material is suitable when weight is premium, or heat conduction is important.  Thin walled tube will still be strong but not as strong as regular walled tube.  It is ideal for lightweight applications and small diameter tube.