From personal experience the 24-70 II lens is one of the best zoom lenses canon make.

Trying to compare a zoom with prime lenses is some what a mute point - from a quality point of view there a many reviews but in the end the 24-70 cannot open up to F1.4 and the 50mm cannot frame a subject like the 24-70 at 70mm.

For myself it is horses for courses - If I am going to do a portrait shot in a controlled environment (inside or outside) where I want the best bokeh then I am going to go for the 50mm but if I am unsure of the FL I will use, then I will alway pick up the zoom. I have never been disappointed with the results.

Canon's trio zooms (24-70, 70-200, 100-400) for us mere mortals are indistinguishable from a prime lens in all but the pickiest pixel peeper.