We're debating inside our team on whether to use a question mark icon, or an "i" icon for prompting a tooltip that reveals more info.
Which do you say conveys the message better? Or, how do they differ in the message they convey?
The convention is that the question mark indicates extensive help is available, provides an interface for someone having a problem to click, and implies that a more sophisticated means of resolving the problem is being offered. The (i) indicates only that some additional explanatory information is available, but not an extensive help system.
Think of an (i) icon as being analogous to no more than the instruction plate mounted next to an operating control on a machine, and the (?) icon as being analogous to a link to the to the entire system's operating manual, already opened to the relevant page.
For your described use case it sounds like the (i) icon is appropriate.
The difference between the symbols as UI element is that, from the perspective of the system, "?" is passive, and "i" is active:
"?", the system offers the information to the user, that he needs when he has a question.
"i", the system offers the additional information to the user, that may be useful even when he does not have a question.
"i" is the right one to use if you have information that
"i" when information is already present and known to the user, but is not visible directly. Indicates general guides to help the user.
"?" when information is already present/ being used and might know it or is uncertain about it, also the information is not visible. Indicates the complete understanding of the basic info available.