You will not go wrong with either one. For portraits, unless you are enlarging the prints to huge dimensions, you probably will not see much of a difference. For peace of mind, if the money difference is not a major factor, get the MkII. It will last you for decades, well beyond your current body.
+1 this is so very true, I have the 70-200 2.8 usm so I guess I don't know what to call it since it's not the mark I but I love my 70-200 so much I couldn't imagine how much better the mark II is but it's your money and bday. Like the post I quoted stated you can't go wrong with the piece of mind having the best! It's certainly a chunk of change but well spent for sure.