I've never used one but I have used the 1Ds mk3 extensively for the last 4 years. This is a full frame camera but I am lead to believe that the 1D mk3 is a crop sensor? the 1Dx is a full frame but the 1D mk4 is crop and a 1Ds mk4 was never produced.
You are teaching me stuff! Challenging what I assumed to be FF. When I read your posts I went here:http://www.visibledust.com/sensorsizes.php
EOS 1Ds Mark III 24 mm approx. 1.0 x
EOS-1D X 24 mm approx. 1.0 x
EOS 1D Mark III 20 mm approx. 1.3 x
Canon 1D Mark IV 20 mm approx. 1.3 x
EOS-1D C 24 mm approx. 1.0 x
Is the answer to your question 'Technology available at the time for two different purposes'? And today they have the X ... FF and Fast.
Makes me wonder what the best portrait/landscape camera in this lineup really is?
?? Clearly the winner for Sports/Action is the X.