As an owner of both, I shoot a ton of sports photos, also. I would much prefer the 1D4 over the 5D3 if I only were able to own one. I do a lot of outdoor sports photos (football, softball, baseball, track, etc.) and I would feel safer knowing if it started to rain, the camera wouldn't hesitate to work for me.
The 1D4 has a certain... feel... to it. That just makes you know you have something, that if you drop it, someone runs into you, etc., that you know you will be able to continue on, should something happen.
And I have dropped my 1D4, two feet onto hardwood floor at a basketball game. Scratched the paint a little, but nothing else. I can't say I would feel the same if I dropped the 5D3.
Eyal wrote in post #15811068
I have both.
The 1D4 is superior in sports. Tracking faster and the higher FPS helps a lot in some cases.
The 5D3 is better at low light, and the colors feel a bit better.
Note that the 1D4 has light metering on the 19 main AF points while the 5D3 is only in the center, so it helps a lot if you want to compose and set he lightning to a certain off-center point.
There is also the extra grip which helps, and its a bit less bricky than the 5D3 with a grip.
If you want one body and your main is sports, I suggest the 1D4. For fires, it depends what you are shooting exactly. If you are focusing on decent light and the fire itself, the low light improvement of the 5D3 isn't that a must have.
Also the fact that fire areas have a lot of smoke, dust in the air, heat, I suggest again to take the 1D4 which has a more sturdy body overall than the 5D3, and might live longer in hard conditions.