Upload & Sell: On
The real question is what will it cost for the alternative. If you have a 1DS3 and sell it to buy a 1DX or 5D3, what will the value of all three be when it is time for another upgrade?
Just sold a 1DS3 and bought a 5d3. Will they be about the same value in 3 years? If so, the 1ds3 is the cheaper solution.
The 1DX will certainly be worth more in 3 years, but of course the upgrade would also be costlier. But the 1DS3 may hold value better than a 5d3.