Simply put, go with the MkI unless you live between 2.8-3.5 and need your images, between that range specifically, to be the sharpest possible. In my experience, having owned and tested both simultaneously, the MkI and MkII are equally sharp from f4 and beyond. As for clarity, color and contrast, the MkII is excellent out of the camera. However, with good PP, you can achieve the same look and feel with the MkI (bokeh notwithstanding, however the differences between the two are greatly exaggerated IMHO).
I kept the MkII because I shoot a lot of high ISO low-light indoor sports, and the difference in sharpness wide open is enough for me to justify the additional cost.
No matter your choice, the MkI and MkII are legendary performers, and have produced some of the greatest sports and editorial images in recent history. There might even be a wildlife or wedding photographer that has made a few pennies with these lenses.