Which one should I choose between IIML-ABM and IIM Shillong PGP?

There are almost the same given opportunities in both the institutes during summer, Laterals and Final placements. Well, there are two things you need to focus upon:

A few selected firms would not accept ABM students for the process (read Top 3 Consulting and Investment Banks and a certain Indian FMCG firm).

You will have to do ABM courses in your 2nd year with an option to take 2-3 electives, whereas in PGP you will be provided almost 15 electives.

Some facts regarding both institutes so that you can make better choices out of them.

If you want to do Marketing or General Management or end up in consultancy, then IIM L ABM is a good course because ABM 2nd year courses are nothing but Marketing, and so you don't miss out on anything compared to a PGP student. But go for IIM L ABM if you want to seek financing or operations.