Which is better for CSE, IIIT Guwahati or NIT Andhra Pradesh?

You can opt for NIT Andhra Pradesh. Now, you may be hesitant about choosing it because it is 2 years younger than IIIT Guwahati. But NIT Andhra Pradesh has both a very good location and faculty. The location of your institute matters since there are few companies that clearly do not visit the North East for placements. Even though IIIT Guwahati does not qualify to be at a very remote location, you will definitely have better options at NIT Andhra Pradesh. 

Here are some details about the institutes.

The 2 years gap does not necessarily make noticeable differences, but the location does. For example, NIT Goa is only 7 years old but still has notable placement stats only because of its favorable location. So, if you want to enroll in IIIT, then either IIIT, Trichy, or Sri City can be considered better than IIIT Guwahati. IIIT Guwahati also had poor placements for CSE in recent years. 

The final decision is yours, but NIT Andhra is definitely a better choice.