I use (i) for InfoTips that popup with additional information based on hover or interaction with the icon. An InfoTip can contain a link to the help system.
I use (?) for Help. Interaction with (?) would open the application help system or web page.
My thinking is that (i) provides additional - albeit limited - information in-context to the item it relates to.
(?) is for when "I have questions" and the user will need more than a limited InfoTip.
Using (?) should be very visually distinct and not positioned near the end of a line to prevent confusion, as a question mark is common end-of-sentence punctuation.
Example from a recent Microsoft Outlook version illustrates this. Note the stand-alone question mark ? in the upper-right of the dialog box. Choosing that icon opens a web page that is the Outlook and Microsoft Office help system.
Each option has a (i) icon, for additional information about that specific item. The popup InfoTip appears on mouse-hover.
