Have both and won't part with either. Weight and size of f4 is its advantage when on the river, trail or daytime walks. F2.8 advantage is the extra stop when needed indoors, dusk and dawn as well as the bokeh for portrait work. Both are sharp, with a slight edge for the f2.8 version. Use the f2.8 mostly for covering events and portrait work.

If your own work is starting to push capability of f4 aperture, then it might be time to switch or add the f2.8. Otherwise there won't be that much difference except lens weight and the lightened wallet. Another option might be an even wider prime or two for those purposes when the f4 just isn't wide enough anymore.