Cyproterone is said to be 4 times stronger than spiro, is not FDA approved in the USA as far as I know, but I think it might be used for cancer treatments.
Spiro is considered much more safe, it is so safe it is even used for skin problems at times! and acne I think and stuff..
Spiro, seems to be very safe and pretty much nothing for the liver to handle so, safety over effectiveness.
Even though it is said that Cyproterone might have better effects even if taken at same doses, I don't think it is completely decided that it is really having better effects, it might just be a feeling.
Spiro is Aldesterone, which is actually a male hormone about 15 weaker than testosterone if I remember correctly.
Spiro is usually not taken at such high doses.
Edit dosage info