Which course should I pursue among B.Tech in aerospace engineering and B.Tech in mechanical to pursue M.Tech in aerospace later?

Dear, It is better to go for aerospace engineering both in B.Tech and M.Tech. Aerospace has better job opportunity after B.Tech itself. Keep that opportunity open also. Best wishes.

If you are sure and wish to go for aerospace in future - go for B.Tech in aerospace and then M.Tech in the same.