From what I can tell HREW is a type of pipe.... Sch 40 relates to the wall thickness of the pipe.
So for example you could get 1.5" Sch 40 in different grades like A, B, F... and in different types like HREW, ERW or Seamless. Plus you can get most of these in a couple of different materials like A-53 and A-106.
Pipe gets a bad wrap because there are low grades of pipe. You just want to use a high grade to make sure you get a strong material.
Grade B is very strong but not intended for close coiling (bending)
Grade A is almost as strong with a Yield Strength of 30000psi and Tensile of 48000psi. Using that with a seamless or ERW type works well for cages. I usualy use 1.5" sch 40 for cages and sch 80 for high impact places like rockers (the extra thickness keeps it from denting).