Hi, I prefer to stay in Beijing. However, Tianjin is very close to Beijing, you could take a inter-city train from Tianjin to Beijing and it only takes about 30 minutes.

Hi,here!
I advice you stay in Beijing.Since its your first time to China,Beijing have many Secnic spots Worth a visit.One day is enough for Visit Tianjin.
I am working as a Beijing Tour Guide for more than 6 years.
Experiences tells me that if it is your first time come here,you should visit the scenic spots of Beijing as follows:
1.The Tian'an men square
2.The forbidden city
3.The Temple of Heaven
4.The Ming Tombs--often for Dingling
5.The Summer palace
6.The Great Wall
7.Visit Beijing Hutong by tricycle.
If you are interest in Cultural art of China,you also could watch Peking Opera and The performs of Chinese Kongfu and so on.
All the scenic spots are reasonable arranged,3 days is enough.