I currently am using a Rebel T3 with my 70-300 L lens. I have been shooting for about 2 years now and find myself mostly shooting outdoors sports. I believe I have come to the point were I have out grown my Rebel T3, but I am not sure if I want to stay with an APS-C sensor or go full frame. I mostly shoot outdoors soccer and baseball. I was wondering if my 70-300 L would perform better as far as auto focus and image quality on a 7D or 5d Mark iii? Also, will 6 fps on the Mark iii be sufficient enough for sports? I thank you all in advance for your time and knowledge! Cheers!!!
Like most things it depends. Do you shoot under lights at night? If so the 5dm3, is going to perform better (autofocus and cleaner high ISO), otherwise the 7d will perform wonderfully and provide greater reach.
As for FPS, in my experience as I get better at shooting most sports this matters less, as I am able to time the shots, rather than gunning through a sequence. If your timing is not great or you want to capture the stop action type sequences, the extra couple frames per second on the 7d will help.
I primarily shoot indoor sports (Volleyball), and have great success with a 5dm2, but it took a lot of learning to get the autofocus to work in this environment. I upgraded to a 5dm3, and it makes getting the shot much easier. I have also used the 5dm2/3 for soccer as well as twice I have shot with a 7d for soccer (with 100-400, not your 70-300). In general I would choose the 7d for daytime soccer for the greater reach.
Everything in life is a tradeoff, there isn't a clear-cut choice - you have to choose based on the factors that matter for you most.
The 5Dmk3 is definitely a better camera than the 7D (except for two things), it has better auto-focus, higher resolution, better dynamic range, and in general better everything - it also bigger, heavier and more expensive.
The two things where the 7D is better are the frame rate and the crop sensor (yes, full frame can be a disadvantage too) - with the 5Dmk3 you will get the same field of view at 300mm like your Rebel at 187mm, if you mostly shoot at 200mm-ish and below that doesn't matter but if you often go to 300mm you will need a longer lens with a full frame camera (to get the 300mm field of view you will need a 480mm lens!).
Like I said in the beginning everything in life is a tradeoff, the 5Dmk3 will take better picture but costs more and may require a new (expensive) lens - the 7D on the other hand will be cheaper and is still a very good camera.
You should also look into the 70D, it has a newer better sensor than the 7D and the same auto-focus system and will be cheaper - but it's not rugged like the 7D or the 5Dmk3.
The autofocus consistency in terms of accuracy from shot to shot is worlds better in the 5D mark III which has a 1-series level focus system. The 7D on the other hand, while slightly faster and much more configurable than other consumer/pro-sumer grade bodies from Canon, is not consistently more accurate than them. I own a 7D and that is my biggest frustration with it. Even when the AFMA is dialed in, a good number of shots just miss correct focus. Some are front-focused, some are back focused. In my case that is with an EF 70-200mm f/2.8 L IS II mounted to the 7D. For more on the accuracy of various Canon bodies, see Roger Cicala's excellent blog article. For more on exactly how the 7D AF system works, see http://photo.stackexchange.com/a/41179/15871. Other multi-point systems, including the 5D3 are similar. To get the most out of them you have to understand the actual 'map' of the focus points and how the area of greatest contrast within the entire active area for any particular point is what the camera will focus on. But most other sophisticated multi-point systems are more consistent than the 7D.
On the other hand, the 1.6x crop factor of the 7D will give you more reach than the 5D3 with the same lens. If you crop a shot from the 5D3's 23MP down to the same size as the 7D's sensor, you will only have about 9MP resolution instead of the 7D's 18MP.
The larger pixels and newer design of the 5D3 also mean better low light performance. Real world testing gives an almost two stop edge to the 5D3 over the 7D in terms of noise at high ISO. And with the slower aperture of your 70-300 L compared to a constant aperture f/2.8 telephoto that is a key issue.
As far as frames per second go there are several considerations:
The 5D mark III costs about twice the price of the 7D. Whether the far superior focus system and significantly better low light/high ISO performance is worth it to you is a question only you can answer. The only other thing you will be giving away is the reach of the 1.6x crop factor of the 7D and a very modest difference in burst rate.
The 6 FPS should be fast enough for sports if you have decent timing. The AF performance will be far better and more consistent on the 5D Mark iii. The main thing you will lose with the 5D over the 7D is the extra reach that the APS-C sensor gives you, but the improved AF points, speed and consistency will be more than worth it if you have the money to burn.
Full disclosure, I am a very happy 5D Mark iii owner that shoots with a 70-200 f/2.8L IS II and used to use the non-L 70-300 f/4 before the 70-200 f/2.8.