"As mentioned, Canon has informed us that the Canon EF 70-200mm f/2.8L IS III USM Lens has the same optical design as the II. That makes predicting the III's image quality easy, especially since I've used the prior version of the lens for over 8 years. However, in your case I think I would go for the 7D Mk II and later get the ef 70-200mm f2.8L II. Sell your current 70-200mm f4L top help pay for it. I don't hold the same regard for the 100-400mil II especially if you buy the 70-200mm f2.8L II, later on. A better option is the ef 400mm f5.6L prime. And, it is cheaper! Then buy the Canon 70-200mm f/2.8 L IS MkII lens with the Canon EF 1.4x III Extender. You did mention the price difference, but don't specifically mention it as a big concern. If cost is not the driving factor then the above stands true. .

canon 70 200 f2 8 is ii vs iii