Ron Guth: All Proof 1887 Three Cent Nickels are overdates, though some overdates are stronger than others. To be sure, the most desirable ones are those that are the most visible. The attribution image above shows clearly the difference between the "Strong" and the "Weak" overdates.
According to the PCGS Population Report (as of August 2011), the distribution between the two overdates is fairly equal. However, it appears that the Strong overdate is found in generally nicer condition, since the most common grade for the Strong overdate is Proof-65, compared to Proof-64 for the Weak overdate. Less than 20% of the Strong overdates are Cameos and, to date, no Deep Cameos have been certified by PCGS. The finest Strong Overdates are 8 PR-67's (4 non-Cameo and 4 Cameo).