I just saw that its only a 12' run. I would use the steel pipe. The fittings for the PE pipe will cost more than the steel. The PE pipe cant come out of the ground so you will need to use some steel anyway.......extending 18" horizontal also. Add that together on each end and you have 36" that must be steel of the 12' run. So you would end up with 9' of PE between steel pipe. makes no sense.