Which city has more attractions, Beijing or Shanghai? Can we purchase train tickets in advance?

We'll be spending 7 full days in China on April but I'm having difficulty on how to divide our time. 4 days in Beijing and 3 days in Shanghai or the other way meaning 3 days in Beijing and 4 days in Shanghai? Which city has more attractions and good for kids? Do we need to purchase our train tickets in advance or can we just buy it when we get there? Thanks in advance for your help!