If you'll be outside, the f/4 is the way to go; but your camera won't AF with the 2x converter on this lens. For that you'd need the f/2.8.

I wouldn't make decisions based on the 2x converter unless you're looking at the big L super teles that take it pretty well (like the 500mm f/4 II). But the f/2.8 with a 2x is probably better than the sigma at 400mm. Check Bryan's ISO crops and compare.