I'm going to purchase some edition of HoME, but haven't yet decided between getting the 12 books (hardcovers) individually and getting the set of 3.

Aside from cost, are there any substantive differences I should be aware of? For example, the 3-volume set was assembled more recently; if that means it has slightly updated content, that would matter to me. Also of interest: illustrations (if one has more, or better, illustrations (if there are any to begin with)), notes, footnotes, other references etc. (I want them).

Related question: is the 12-volume set 12 volumes of history + 1 index (13 total books) or 11 volumes of history + 1 index (12 total books)?

FYI, I know there have been varying dustjacket designs for the individual hardcovers; I don't care about those and they wouldn't factor into my choice. Also, if I go the 12-volume route, I would be picking them up second-hand and not POD through tolkien.co.uk.