I have to say it's going to be the colour.
The colour happens to have a lot more detail that just wouldn't really fit in the black and white, I think. Things like the darker section on Rill's head, the fact her eyes are yellow (and the shine and highlight across them), the detail in the fur, the depth in the face...
It has so much more in it, I have to pick the colour. The black and white is very clean, but it's almost too sterile... it's pure black and white, with no shading or tones. It seems almost like it would be a fair comparison if you did flat unshaded colours versus that. The detail on that is very pure, but it's not really as appealing as the colour.
I'm going to regret this opinion when anything mechanical shows up, however, and I reserve the right to beg and plead until I'm blue in the face for black and white versions of those with all the fiddly little lines and details intact, please.

I loves me my mechanical bits.

Anyway, that's my £0.02 - take them as you will. (None of those bloody US cents or eurocents for me!

)
Also, this is a day I'm agreeing with Kao. Mark it down, lads 'n lasses, it's The Beginning Of The End.