Hello,
I have given both the IDP and the British Council exam.From my experience i can tell you that the British Council is better managed(Atleast in mumbai).They have the exam at three centres while IDP conducts the exam at one location and it gets really crowded.
There is a notion that it is easier to score in the IDP as compared to the British Council.I did not find that to be true.Also British Council allows you to Send your scores to five academic institutions free of charge.IDP does not provide this option.
all in all i can say that the British Council is a better bet than IDP.
I hope it makes sense to you.
All the best for your exam.