[Rojodiablo]

I am split on my thoughts here; the SS will hold up better. Steel in the ground will get attacked quickly by the minerals in the soil. SS would not last forever in this condition either, but it would last longer.
If you did the SS box, then I can suggest after having done a lot of mig on SS that you will get a decent weld out of it with the C25. I would recommend you polish the welds when done; it will help leave a finished surface that will fight corrosion better than just welding it and putting it to service.
Other options are to get some Herculiner urethane and coat a steel box well with it, several coats. It holds up to the elements very well FWIW.