Which is better, ECE at IIIT Allahabad or Civil in IIT Guwahati (interested in coding)?

The electronics and Communication Engineering branch of engineering is not limited to coding but deals mostly in hardware than software.

Likewise, core sector engineering also demands coding skills. You will require it while analyzing structures, thermal and flow modeling, vibration, etc.

Following are some recommendations for you:

• Follow your passion. Choose the branch that you are passionate about. If rectifiers, diodes, microprocessors, amplifiers, microwave, power electronics, etc. excite you, go for electronics. If mechanics, structures where you can yourself make giant elements interest you, civil would be the right choice.
• Placements at IIIT will definitely be good but, an IIT will always be an IIT, both in the short as well as in the long run. As a civil student, you will get an opportunity to opt for a minor at IIT Guwahati. This gives you a chance to study electronics and communication along with civil. Even during the placements, you can sit for the top software companies or consulting or banking jobs. Hence, IIT Guwahati is a fair choice even considering the short term.
• After two to three years of job, most of the IIT graduates go for an MBA, MS, Civil Services, Engineering Service, or join as a faculty at some top coaching institutions. Wherever you go and whatever you do, the tag of an IITian will always remain with you and will definitely benefit you.
• This might be quite surprising but many talented people who at one point in their lives dreamt of starting their own company, becoming the next Elon Musk or Steve Jobs go the central government jobs. They end up joining civil services or engineering services or some other central government job. After a certain point in time in life, 99 percent of the youth are attracted by central government jobs. 

To be honest, if you want to go for engineering services or similar central government gazetted level services, civil engineering would be the best choice. Since you are interested in coding, I am not sure of the work satisfaction that you will get working as an electric engineer for a core central government job.