Any difference between 早安 and 早上好? Is there any difference in meaning, formality, tone, or otherwise between the "good morning" greetings 早安 and 早上好? Or just regional preferences? (I'm thinking perhaps the former may be more common in Taiwan and the latter more common in Mainland China and/or northern China but I'm not sure.)
From the accepted answer at Baidu:


  Both are words of greeting. Nowadays people generally use “早上好”. “早安”
  will give a feeling of before 民国 (the Republic of China (1912-1949)), 
  or a literary feeling (it's common in literature).


Other people think 早安 has a warmer feeling.

Of course this is the perspective of mainland people.

In Taiwan 早安 is used much more than 早上好. （Ratio: 28, Google advanced search: exact phrase, country TW, language zh-TW)


"早安" 5,340,000
"早上好" 192,000


In the mainland, incidentally, 早安 is also used much more than 早上好, although the degree of usage seems much closer. （Ratio: 2.3, Google advanced search: exact phrase, country CN, language zh-CN)


"早安" 24,500,000
"早上好" 10,400,000