How to choose 5d3 and 5d4?

It comes down to money. You have to understand that the real cost of the camera is not the price at the time of purchase, but the degree of depreciation in the process of use, that is, how much money will be lost if it is sold second-hand after three to five years. Although 5d3 is cheaper than 5d4, after three years of use, the depreciation of 5d3 may be greater than 5d4, because it is too old. Even if the depreciation of the two is similar, your experience will be very different in these three to five years.