I think it depends upon what you will be photographing and in particular if you will need to focus it in low light. Whilst the slower lens is better quality around the edges at low apertures, it is much easier to focus the f2 model in poor lighting. I use mine exclusivly for interior shots (as I use my Nikkor 20mm f2.8 AIS lens). Having soft edges is not always a negative attribute - it can be creativly used to focus the viewer's attention on a key subject in the photograph. If you ever venture into large format photography, wide angle lenses usually require a centre filter to avoid the fall-off in lighting around the edges of an image - however these filters are optional. If you are photographing a subject that needs even illumination in the print, then you can add the filter, but by not using the centre filter you can draw the viewer's attention to a key part of your final print. If you think of the Nikkor 24mm f2 in this way, it might help.
Ideally, I'd buy BOTH.