Which is more common: 川普 or 特朗普?
Haha, funny question. "特朗普" is the official transliteration, used most commonly in official media of China, such as CCTV（新闻联播）and People's Daily(人民日报), while "川普" is more often used in social media or among people's casual talking. 

"川普" is transliterated based on the pronunciation of "Trump", which is reasonable. However, "特朗普" has been used as the transliteration for "Trump" for a long time. According to 《英语姓名翻译手册》（新华通讯社译名室，商务印书馆，2004年）, which is an authentic book for transliterating English name, Donald Trump should be transliterated as 唐纳德。特朗普. In mainland China, the official medias usually follow that rule, so they usually use "特朗普". Other media, such as VOA or Taiwanese media does not necessarily follow this rule. So, the use of "川普" is also correct.