Q: I have a project in mind, but don't know whether to hire an architect, or a structural engineer. Can you advise me?

A: The structural engineer is involved in the design and supervision of structural constructions, though their expertise is in calculating loads and stresses, or analyzing the strength of materials in structures such as foundations or beams.

It's probably more accurate that both an architect and a structural engineer collaborate with each other.  The architect is more concerned with the aesthetic elements involved and broader issues (sustainability, neighborhood or environmental impacts), while structural engineers are more involved with the scientific aspects of the construction.  It'd be safer to say that it's not a black and white type of picture, and that the skills of these professionals have some degree of overlap. Therefore, the architect is quite able to do stress load calculations, and some structural engineers may have an aesthetic design sense.