IIT Bhubaneswar is the better option in this case. Civil engineering from IIT Guwahati has less scope as compared to ECE from IIT Bhubaneswar. Although IIT Guwahati is in the category of old IIT’s unlikely IIT Bhubaneswar and also comes at 7th among the engineering colleges in India.
IIT Bhubaneswar offers better placements for electrical and electronics & communication engineering. In 2019, electrical engineering had an 86% placement rate and the average package was around INR 11.44 LPA at IIT Bhubaneswar. At IIT Guwahati there were 77% placements for civil engineering.
If we compare on the basis of streams; Civil Engineering lacks behind ECE. Civil is easier than ECE but the decision depends on your interest and whichever course interests you more should be the first priority.