I would sayAmrita. The reasons are as follows

Assuming that you are getting into coimbatore campus ,this campus is far better than Sastra campus . ( I have been to sastra twice for thier fest and I have stayed there . Sorry to say that thier amenities are average level . I am an alumni of Amrita and I know how good is Amrita campus )

Weather is perfect in amrita coimbatore whereas Thanjavur is so hot . (Thanjavur is my native )

Placements is good at Amrita . Even sastra has good placements . But on a comparative note Amrita gives you more options (I got into 4 companies when I did my engineering)

The brand name ofAmritais far better than Sastra. The alumni from Amrita has done a good job in developing a brand for Amrita . This helps in your career and in your resume

Academics , I mean the course wise Amrita is better . The syllabus and the exam patterns are in par with Tier 1 colleges like IIT

Grading system of Amrita is in relative format . And the courses are offered based on grade points . There are elective options too

Amrita is better         .