I have an 842 on my system at the house. The 642 is nice, we install them into commercial systems, but for what I'm using it for, the 842 is fine.
The 642 is a bit more solid and allows for easy pressure settings as it has a dial on the front. The 842 you have to use a screw driver, but I don't find it troublesome.
You can't go wrong with either.
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