I will suggest you to opt maths in your msc course.

For pursuing M.Tech afte M.sc the only best way is to go through GATE.

GATE (Graduate Aptitude Test in Engineering) is conducted by IISC in collaboration with IIT's every year, for providing admission into M.Tech, MS and integrated P.hd (Offered by CSIR ) at IISC, IIT's and NIT's. Hundreds of institutes other then these except GATE scores, even GATE scores help in getting Diredct jobs in MNC's and PSU's also.

For getting into IISC, you have to score at least 640+ score alongwith at least 96% Percentile and good AIR, because IISC only 20 seats in its M.Tech.

Where as for IIT's one can compromise with AIR, but for getting Direct admission criteria is same. For Admission through Interview one with 500+ score can be wait listed for admissin through Interview. Where as for MS score of 500+ and 95% GATE percentile can fetch a seat in MS there.

For NIT's score of 400+ and pertile of 85% can fetch you a seat there.

Feel free to ask me in the comment box if you are having any further doubts

hope you find it helpful

Good Luck!