You are always going to get both whenever you have two or more slits of proper width. The diffraction manifests itself by the fact that the radiation from a slit is as if the slit is a point/line source. So no matter what the wave is behind the slit, be it a spherical wave, plane wave, cylindrical wave, etc, a suitably small slit will reradiate the wave as if it was a point source.
Interference results in the constructive and decontructive interference patterns. This manifests itself as bands/points of maxima and minima along a plane parallel to the screen. The characteristics of the slits and the distance at which you observe the resulting intensity patterns will dictate the how the interference manifests itself in terms of where you will find the maxima and minima.