If you think 300 will be enough, I'd say save some money and go with a 1.4 convertor for your 70-200. I think the image quality will be as good, if not better, that either of these lenses. Your AF
will slow down a little compared to the lens without the convertor, but then neither the 100-400 or 28-300 are noted for fast AF
If you want the wide angle benefit of the 28-300, then use the money you save to get (for example) a 24-105 or 17-40.
I you NEED 400mm, then you don't have much choice. I wouldn't rely on the 2x convertor on the 70-200
Colin K. Work, Pixstel