words - 私は米国人です vs 私はアメリカ人です

They both mean I am an American but which is preferable in everyday speech and in writing, and why? Is the former more formal than the latter?

Answer

Today people never say 米国人 in everyday speech and even in written form most people express American as アメリカ人.

The obvious exception is on a newspaper. Writers there still express it as 米国人, for possibly two reasons:

1. 
   

   米国 is shorter than アメリカ.

   
   



2. 
   

   Kanji is more suited than Katakana to vertical writing, which is still used in newspapers.

   
   

I also want to note that this tends to be applied to 英国/イギリス as well. Also, Kanji is often used to express each country (e.g. 仏, 独, or 伊 to express フランス, ドイツ, or イタリア) even on the web news just because those are shorter.