I have been thinking of replacing both 7d and 6d with a 1d4. I use the 7d for amateur sports/high shutter clicking events and 6d as a 2nd/backup camera to 5d3 for weddings. The good thing with my current setup is all cameras use the same batteries and I have tons of them. The 6d is a very capable up to iso3200 camera. The bad is the 6d's focusing is miserable in low light. I did a direct comparison on focusing speed with a friend's 1d4 and the keeper/in focus rate of the 6d is close to 30% compare to 90%+ for the 1d4. But the 1d4 high iso is only good up to 1600 based on my rough estimate. Also the 1d4 uses a different and more expensive battery so I have to invest extra on it. Relative to the 7d, the only difference is the 1.6x crop, which is not a big deal to me. In addition the 1d4 has dual card slot, which gives me the extra peace of mind.
What would you do if you are in my situation?