To me, if you have to ask, you've never shot with a 1 series Canon camera. I have both and both fill a need. The 5DM3's silent continuous shooting mode alone makes it a more appropriate camera for some subjects and events while the 1D X is a vastly superior camera in just about every other way. One thing that people overlook about the 1D X is the accessory difference. Both the WIFI and GPS accessories take power from the camera so no bag full of AA batteries. I have a GP-E2 GPS for the 5DM3 and it works great for GPS coordinates (not so great for direction...) but it requires AA batteries and it sits in the hotshoe. Yes you can use it with a USB connection but it's in the way. The 1D X accessories are smaller and attach to the left side of the camera. A plus for me besides the obvious camera build quality, built in vertical grip, weather sealing, better AF, higher frame rates, etc... of the 1D X. It was tough to make the decision to sell the 1DM4 to partially finance the 1D X but, in the end, I'm glad I did.