This_Is_Cinerama said:
You have a 6D. You have never felt completely satisfied with it for many reason. Reasons that could include 5DmkIII envy, CF, focus points and overall build etc. It is personal and may or may not make sense to everyone.
Dual card slots on the 5D3 are one of the BEST features of this camera, albeit limited in speed on the SD slot. That said, I usually have it configured to shoot to both cards simultaneously and if I really need the speed I can click a couple of buttons and remove the SD card as a bottleneck to the buffer.
Have you checked that you have the fastest available SDXC card? It should read at least 600x next to the class 10 UHS-1 markings. Slower class 10 UHS-1 cards work fine in the 6D, but the speed difference is noticeable when the camera saves multiple images or HDR is used.
Don't feel that this would be wasted if you buy a 5D3 with an even faster CF card, there is also a second (SD) slot that I'm sure could be useful for backup.