Should I opt for IIIT Guwahati CSE or should I take mechanical at DTU. I am a Delhite, but CSE is my preference. I have 19,557 rank on the JEE Main?

If you like CSE, then you should take it without any doubt. But before making such a big decision, it is important to know the aspects of both. Make sure to study about these branches and take advice from seniors.

• Talk to the final year student of your preferred branch of both the colleges because they have all the knowledge about the real situation. They know about the final year projects and the placement scenarios. 
• It is also important to know if anyone in your preferred branch has done any innovative work. Don’t get misguided by talking to irrelevant people. 
• Make sure to get all this information from reliable sources. 

It is a crucial phase, you should not hurry up. Instead, take time, know all the facts and then decide on what they want.

One of my friends has studied at DTU and according to her, the atmosphere is amazing there. Everything from Fests, seniors, college life, faculty, the crowd are way better than IIIT Guwahati. In terms of location too, DTU has the advantage of being situated in the centre of Delhi. 

You should definitely choose Mechanical at DTU without a second thought. However, if you are worried about branches, then both branches are good.