Well, it depends on your interest. Both fields have good potential, but structural engineering is high technical field, so my suggestion is that if you like deal mostly with technical things, structural engineering is best field.
While on the other side construction project management has also various opportunities, and after the completion of M.Tech in construction project management one can work as a project manager, analyst, project consultants etc.
To conclude, both fields have its pros and cons. My suggestion is that if you want work in technical areas, in that case structural engineering is best. Considering the job opportunities, construction project management has better on campus job placements compared to structural engineering. However, it is also not much difficult for structural engineer to find a job off-campus, but there are limited large MNCs in structural engineering compared to construction project management.