The MII has some benefits:
1) Improved optics - whilst the original was no slouch of a lens by any means there was room for improvement, esp at the 200mm end and when used wide open at f2.8. The MII makes these changes and the improvement is noticeable in shooting. Myself I've experienced this directly when moving from the MI to the MII - before I couldn't effectively use a 2*TC, it would work but quality was only so so - with the MII the 2*TC gives you an effective 140-400mm that is pretty much on par with the 100-400mm L
2) Improved IS - the original was generation 2, whilst the MII has a generation 4 (I think) IS system. In practical terms this means you gain around 4 stops worth of shutter speed with the IS. The engine is also much quieter than before
3) Improved magnification ratio - best seen with teleconverters and/or extension tubes; but the new lens makes a decent close up option
4) Slightly lighter - mostly minor here, but it is a tiny bit lighter.
In general either one is a professional top grade choice, the MII is just better. It's really a case of deciding for yourself on if you want to jump right for the best there is or if the older one is enough for your needs. The second hand price of the old one has remained high, showing that whilst people are upgrading there is still a demand for the quality of the old. Heck myself I only upgraded because of the afore mentioned desire for pushing the lens twice its native focal length - had I been only using it in the 70-200mm range chances are I would not have upgraded.