Hello ..

Humanities have traditionally been termed as the bastion of civil services examination. If we look at data released by the UPSC in their annual reports, we can see that humanities subjects have had a higher success ratio compared to others. 

For IAS preparation it would begood if you pursue your BA in the subjects like History, Public Administration , Economics , Constitution & Geography. And among these I think Geography & Public Administration are most popular optional subjects for the mains and account for the 50% shares in the final mains result.

I think you should go for humanities stream. 

Hope it helps. 

Best of luck.