Which one should I consider between an MS in Statistics from an Ivy League college in the USA or an MS in Statistics from ISI Kolkata?

It is important to collect a good amount of information as to how to apply to a US college and then ask yourself which course to consider. After this, you must look for that option that fulfills your requirements and needs. When you qualify for those exams, then you must consider which one to choose.

• For the Ivy League colleges, you will be required to appear for GRE Mains and TOEFL. 
• In almost all cases it would be a better prospect for you to give GRE as an exam as it will give a bit more of a specialized vibe to your application. 
• The next step would be to send in your application to individual universities along with your CV, mark sheets, and letter of recommendation from your college faculty.
• In some of the cases, they also require you to send a good SOP. 

Now, you must gather information about all of the above-mentioned exams and start preparing for them.