This is VERY difficult. I voted Sony, because I think the Alpha 900 would be my choice and I don't think my finances would run to a Nikon D3X. It depends entirely on how much money might be available, and on prior knowledge of the performance of the cameras. On paper, I would have voted Canon, because the 5D MkII ticks all the boxes - native file size larger than Alamy/Getty etc require for stock work, video capable, full frame, LV with magnified manual focus, compact body not huge lump, high-res rear screen, CF card storage, medium price.
But I have had the experience of testing Canon lenses and actually using Canon a lot over the last year (far more than ever before) and there is no way I'd ever buy into the system. The lenses are even more of a lottery than Sony for quality - the difference is that the Sony/Zeiss lenses seem to be either excellent, or faulty - the Canon lenses (even L glass) are just inconsistent so a lens may seem to be perfect but just lack proper sharpness (or whatever). And, even though the high ISO results can be better, I just don't like the 'look and feel' of the resulting pix as much as I like what the A900 produces.
I would not know this without having used the cameras, so my reply is not really valid - if I had no prior idea, I would assume Canon was equal to or better than Sony because the brand in more popular.