This is highly dependent on your choice of function space. As you are using the degree=2 argument in the expression, this will be interpolated into a second order function space. However, if your space V is a first order space, your second interpolation will reduce the accuracy of f.
Similarly, projection and interpolation does to different kinds of approximation: interpolation sets values exactly at degrees of freedom, minimizes the integrated difference over the mesh.
For me to be of any further help, you would have create a minimal code example, as described in: Read before posting: How do I get my question answered?