I had the 300 f/4L (non IS), the original 100-400L, and now the 100-400 II.
I loved the 300 f/4L, but wish I'd had the IS version (both for the IS and closer focus).
I found I wanted the versatility of the zoom, and the only disadvantage (vs the 300L) I found in the original 100-400L was a stop slower speed. I had a very sharp copy of the zoom and didn't see much difference from the prime in real use, but when photographing small creatures, wished I could focus closer (true with both lenses).
Now the 100-400 II is sharper, faster focusing, has IS, focuses down to just 1 meter (this is the biggest reason I bought it), and I think it has better bokeh vs the other two I had. The 100-400 II also takes the 1.4x III very well. The images are very usable with my 5D3. I do have to clean up some CA with the 1.4x though.
Unless you need the speed and only use 300mm, I'd go for the new zoom.