Is it possible to do a day trip from Shenzhen to Guangzhou ? What train should we take ?

Of course a one-day round trip to Guangzhou is available. Bullet trains are frequently scheduled between the two cities. The C trains leave from Luohu Station, and it takes around 1.5h; while G trains depart from the North Railway Station and the duration is only half an hour.

Would it be more convenient to travel to guangzhou by C or G train ?

Both are convenient enough. It depends on what your destination is. And usually the G train costs a little higher.