Chaozhou to Xiamen OR Shantou to Xiamen?

l got different answers from several parties about traveling to Xiamen by bus from Chaozhou or Shantou. Actually, which is more convenient and shorter time to Xiamen, from Chaozhou or from Shantou? Are the buses very frequent from both places to Xiamen? l would like to reach Xiamen by 11am so as to change the fast train to Shanghai. Do l need to purchase train ticket in advance or l can buy it when l reach Xiamen?