Depends on what you need. If Bokeh is your thing, go for the F2.8. If "affordability" (I say this lightheartedly as really very few, if any, Sony lenses are actually affordable in the layman's sense) is your thing, go for the F4. The F4 is very sharp, even wide open, and can get you good results. Also, with the F2.8, you will e able to get clean(er) low-light images, but the A7S can already do this, even at F4, and if used at F2.8 you can get very shallow DoF which may not be to your liking, resulting in you wanting to stop down anyway. The 70-200 GM looks to be a very promising lens, but at the astronomical price Sony is asking for the 24-70, chances are the 70-200 won't be within most photographer's reach. Get the F4 if you need a 70-200 now — you won't regret it — or get the F2.8 if you absolutely need F2.8 and have the money for it. Even at F4 you can already get nice shallow DoF...
The 1.4x and 2x teleconverters do not interest me (personally) as I don't shoot wildlife very much, and have no interest in it. If I wanted to shoot wildlife, however, I would be more tempted to use something like a A6300 for the crop factor and faster burst rates and AF, not my full frame A7r II. It definitely is nice to have the option of slapping a teleconverter on and getting a longe reaching lens, but that comes with the expense of light loss and sharpness decrease. If anything, using a crop camera body is better than using a teleconverter, in my opinion. The extra stop of light afforded by the F2.8 is nice in those times when you need it, but then again I myself rarely ever need F2.8 — F4 at 200mm has shallow enough DoF already... Go ahead with the F2.8 if you can deal with the price and added weight, but if you're looking for a high quality zoom that is available now, and is relatively affordable, go for the F4. That said, however, the new F2.8 supposedly has better weather sealing than the F4, and possible faster focusing motors. If that's your thing, go for the F2.8. You'll have to decide for yourself what you need most.
