Hey, I've been looking to get the 70-300 L, recommended by people on this forum a few weeks back. I have a 5dMkii. I'm not all that familiar with all the terms related to photography, could you please explain what you mean with 'lack of AF sensor type access'? Won't the lens work with all AF points in the Mkiii?70-200 II. It excels at sports and portraits. For games under the lights, the max aperture advantage over the 70-300L is significant when you're trying to freeze action in low light situations. The 70-200 performs OK with the 1.4x III: IQ degrades slightly and AF speed takes a hit but gets you to 280mm at f/4, which is still a stop faster than the 70-300L. The 2x III makes it notably soft (viewing at 100%) and AF speed takes another hit as well, so I wouldn't recommend using it with the 2x unless you really have to.
The 70-300L is good when there is enough light. It's more compact and weighs less (~1 lb less if although it still weighs about 2.5 lb), which makes it a better choice for places like zoos or day trips. If you're ok with the weight and cost of the 70-200 II, then get the 70-200 II. The 70-300L is a great choice if you're budget limited or travel a lot and like to travel light because one lens can cover most of your telephoto range.
Most importantly, you get access to more AF sensor points with the 70-200L II IS lens on the 1D4 and 1DX. You can shoot sports very easily with the 70-200 whereas the 70-300 performs rather poorly due to lack of AF sensor type access.
Sure! Ok, let's just say the 5D Mark III has a potential of 61 AF points and AF sensor types that can be used. The # you can actually use in certain situations is dependent on the lens being used as well. More importantly, the PATTERN used depends on the lens. The 70-200L II IS lens is in Group A, where all 61 AF points can be used in Autofocus mode. This pattern, however, includes dual cross types and cross types. This is superior in sports/action motion. The 70-300L is in Group E, where again, all 61 AF points are available. However, the pattern here is only certain or a few of the cross types. So its pattern is far inferior to the 70-200L IS II lens for action/sports/tracking, etc. I tried to type this off the top of my head, so maybe someone can cite sentences from a manual or do a better job than what I have explained.