My general rule of thumb is to stay on the iOS that the phone came with, for the best speed. I'll update to iOS 8 if they speed it up to almost iOS 7.1.2 speed and when some bugs are fixed.
This ^
Great advice. I only update months down the line once I am absolutely positive that the iOS update doesn't affect speed or battery life. I am perfectly fine with keeping whatever iPhone on the iOS version it came with for as many years as I am using it.