It is hard to do pro and con on a lens not available yet. By the previews it looks like both have good IQ. So the difference will be 100mm longer with the 400, wider 2.8 aperture which might give a bit better Bokeh at 300 with the 300.
The 400 looks promising, I would suggest waiting for some reviews before you commit. To many unproven unknowns like focus speed, Bokeh, IQ ......

you could also consider a 500 mm f/4 Is I series used, it would be just a bit less money and is an excellent lens.