Both Shillong and Guwahati have established centers of Educational Significance, with IIT at Guwahati and IIM in Shillong. Shillong is a far less populated area, not as well connected to the ROI in terms of Railway, Financial & Industrial Ecosystem , when compared to Assam's Capital Guwahati. Incase, youre really serious about UPSC Preparation staying in Northeast, I would rather suggest you to prefer Guwahati over Shillong, if youre from Assam or Shillong over Guwahati if youre from Meghalaya. Both are peaceful places for UPSC Preparation, however you may just have access to better Public Libraries, Coaching Institutes, Internet and Web Services as well as UPSC Study Circles if youre in Guwahati, Assam.