Another variation is that some game came with wood siderails and others came with metal

The metal siderails were used on playfields that had a warp in them that WMS was trying to correct.

I prefer the wood ones. I had a BSD with metal siderails, and a previous owner stripped a few holes, and there was no 'easy' way to fix it. I tend to put new oak side stiffeners on my games as I restore them, using the originals to make a template, so if you've got a playfield with a slight warp.. I'd recommend just using a better grade of wood than WMS did :) It'll make a nice difference.