Canon EOS 1D X Feature Advantages Over the EOS 5D Mark III
More advanced metering system - 100k pixel, 252 zone RGB vs 63 zone iFCL
EOS iTR AF (Intelligent Tracking and Recognition AF including facial recognition
Frame rate is 2x faster - 12 fps vs. 6 fps
Higher frame burst rating - RAW: 38 vs. 18 rated / 54 vs. 33 tested
AF point-linked spot metering and Multi-spot metering
Faster AF driven by more powerful battery pack with select Canon L lenses
EV 0-20 Metering range - vs. EV 1-20
Higher native ISO settings available - 51200 vs. 25600
Higher expanded ISO settings available - 204800 vs. 102400
Less high ISO noise
Faster X-sync speed - 1/250 vs. 1/200
Dual DIGIC 5+ processors plus DIGIC 4 processor dedicated to AE functions
5D III has a single DIGIC 5+ processor and a non-specified AF-dedicated processor
Higher battery life rating - 1,120 vs. 950 shots
Higher shutter durability rating - 400,000 vs. 150,000 cycles
Higher viewfinder magnification - .76x vs .71x
Shorter viewfinder blackout time
Viewfinder provides more nose relief from LCD less nose spots on LCD
Has a viewfinder shutter
Built-in Ethernet Port
More advanced self-cleaning sensor
Better Weather Sealing
Built in vertical grip
Accepts optional focus screens
7LCD brightness levels - vs. 3
More custom functions - 31 vs. 13
Canon EOS 5D Mark III Feature Advantages Over the Canon EOS 1D X
Price - a big advantage for most of us
Higher resolution - 22.3 mp vs 18.1 mp
In-camera HDR mode
Headphone jack
Lighter weight with lighter battery
Compatible with Canon wireless infrared remotes such as the RC-6
Much quieter silent mode