What is the difference between 40125S and 40125W?

and whice one is the best?

Originally the main difference was that the S wrote in Z-CLV and the W in P-CAV. However, with with the latest firmware the S now write in CAV as well.

They have identical hardware and you can actually apply firmwares to swap back and forth between which type your burner is (keep in mind you void warranty - that is if they find out).

I would have to say at the present the S is better solely based on the fact that there is an official firmware update for it that gives is Mt. Ranier support. We all know that the W will have that same update in it’s next official firmware, but like I said this is at present.

Making someone else pay for your mistakes is deplorable. You screw it up, you should pay for it. That’s like dumb kids that don’t know how to overclock their CPU/graphics cards. They end up frying their chip and trying to RMA it.