I currently have 3 Nokia N9-s. All, black, 64 GB and made in Finland. Northen Europe versions. On one of my devices coating started to peel off after I dropped the phone to floor and as it started to look ugly I changed casing to white one (9 months after purchase). Whit casing still looks good, I have not dropped the phone anymore and on the white casing coating does not start to peel off. Phone has minor scraches on silver rear cover (I used phone without any protective casing)..

My other two devices I bought second hand. They seem to be the same age that my first phone. However, both of them have been kept in the original rubber casing and therefore there are no peeling issues. However, there are signs of wear from the case rubbing against the phone.

As a conclusion, white casing does not wear so much over time. However, phone looks way better with good condition black casing.