Comparing the rings, the Mahle rings look to be a better option than the Goetze. The faces are both described as "tapered." The Goetze rings are phosphated and, while I couldn't find a spec, the Mahle rings look to be chrome plated. That's better for the Alusil cylinders, from what I read.
Another difference is that the Mahle rings smaller circles than the Mahle. This should mean lower tension in the bore and less sliding resistance.
Here's a few pics for reference showing the Part Numbers and rings.
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1980 928S Euro x2
1987 944 N/A
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