Which institute is better for a PhD in economics (in terms of exposure, curriculum, study methods, etc.), IIM Ahmedabad or IIM Bangalore?

IIM is a brand in itself. IIM Ahmedabad and IIM Bangalore are prestigious institutes. 

Both are equally good for a Phd in Economics. You may choose either of the two for your higher studies. You can also consider Jamia Millia Islamia for a Phd. A Phd. from this college is also highly valued.

Look into the research interests of the faculty while making a choice. This is the most important factor that you should consider while choosing a college for your Phd. 

Both IIMA and IIMB are a good choice for your higher studies. You must choose between them based on your goals. 

IIM Bangalore is the better option for a PhD in Economics. They have an excellent Economics department. The faculty members actively pursue research projects. Many of the faculty members recently published their papers in the Journal of Economic Theory. IIM Bangalore is especially good for applied Economics. The institute has a vibrant research culture and an excellent faculty base.