What should I prefer, CSE at IIIT Lucknow or mechanical at MNNIT Allahabad?

For placements, IIIT Lucknow will be bast. But if you are interested in Mechanical then MNNIT Allahabad will be a better option. Although you should keep in mind that from the future perspective Mechanical will be a risky choice. Jobs are very less in the core sector and the package will be less. The competition is also quite high in both the public and private sectors.

Here are the recent placement statistics of the institutes.

But in the case of the CSE branch, you will surely get a job with a decent package. You can also change jobs between different companies as there are many opportunities available as IT is a booming sector at present.