buying the 45mm if you already have the 30mm

That's one of the beauties of photography; the fantastic standardized system that is the f-stop.  For a given focal length, aperture and media size (film, digital sensor) the angle of view and depth of field with be the same.  The character of the bokeh might be different (Hard edged orbs, soft edged, etc.) but the not the DoF.  The lens I used is the same focal length, but 1/3 stop 'dimmer', so if anything the NX lens will have a slightly shallower DoF.