Well, both of Guangzhou and Beijing are worth visiting. I suggest you go to Beijing, the capital of China. There are lots of attractions here, such as the famous Great Wall, the Summer Palace, the Forbidden City and Temple of Heaven, etc. You can refer to Beijing Tours(https://www.travelchinaguide.com/package/beijing.htm) which may provide you the detailed information of travelling in Beijing.
Anyway, if you want to go Guangzhou, please check Guangzhou Tour Packages(https://www.travelchinaguide.com/package/guangzhou.htm) to find useful information.

Oh, it takes about 21 hours to travel from Guangzhou to Beijing by train. You could check Guangzhou Train Schedule(https://www.travelchinaguide.com/china-trains/guangzhou-schedule.htm) to know about it.