There isn’t any valid reason for choosing IIM Shillong over the newer IIMs. This is because their establishment time is pretty much the same and not much can be compared. Rather than comparing, I can list down some points as to why you should choose IIM Shillong
The alumni base however is very small obviously because it’s a new IIM. I don’t see why you should go for IIM Shillong and not any other second-generation IIM. But if you compare it to the baby IIMs that have been recently established, it’s definitely a better choice