I am a center-point shooter who for the most part is happy with my 60D/6D setup. I am not totally thrilled with the 6D/60D center point and AI-Servo when it comes to action stuff. I am using decent glass: had Σ70-200 OS, now EF f/4 IS, and the 100 f2. I still get quite a few OOF shots. I've been using cameras for almost 50 years, so I am inclined to believe my technique is acceptable. But, I'll allow that it could be me. That said, I am considering a used 7D or a used 1D3. The prices are close enough so as not to be the deciding factor.
The way I see it the advantages of the 1D3 are 1.3x crop factor and 10 fps. The 7D wins on size and it uses batteries I already have, but also 1.6x and only 8fps. The 1.6 vs 1.3 is not huge, as I've got a 100-400 that will equalize the difference. I am not comparing low-light ISO 3200-6400. I have a 6D for that. Basically, I'm looking at getting useable 8x10 results from my grandkids' outdoor activities.
Should I consider a 1D3 or 7D, or just stay with what I've got, and practice my action shooting? Can I get more out of the 7D/1D3 AF systems? The $700 or so for the camera is not a big deal. Or perhaps I am overlooking something?