Upload & Sell: On
The MkII is probably a better lens, primarily handling-wise.
So if you've got an ample gear budget, then go for it.
It is not likely at all that AF on MkII is somehow inferior to MkI. If anything, Canon might have accelerated the AF drive response of the bare lens.
I have a copy of 300 f/2.8 IS MkI now that I like a lot, therefore, I am really not interested in the MkII, however, setting the price considerations aside, the MkII should be one wonderful lens.