Cast vs Fabricated - pipeline gate valves (M&J vs WKM)

[NPS1978]

I'm wondering what are the advantages in a fabricated valve, such as M&J or Grove's API 6D gates vs a more traditional cast valve. The price difference is substantial but the function is the same. I have a good understanding of castings so I cannot see why there is a need for welded plate valves, especially considering so many of the fabricated valves are rated at ASME 150#.