Sold my 1DMKIV but miss it loads and will now start saving again for another body.
With the 1DMKIV features i think you would only need a 5D MKII for full frame IQ as the MKIV covers the 5DMKIII and some in frame rate,A/F and buffer size not to mention built in grip and better weather sealing.
On the other hand the 5DMKIII does look an excellent camera and a better value buy than the 1DX and if i went that route then would more than likely get a 1DMKIII as backup or even a 7D must admit thats a tough call on your side and enjoy what ever ones you decide on but i do prefer two cameras rather than one all rounder.
Have sold my 300 now but found the MKIII 2x tc gave better contrast and sharpness against the older tc but no improvement in A/F unless used with the newer big whites.