I think I am right in saying that the IMC requires renewal every two years and the IR every year?
Also the IMC can be renewed by most club CFIs, whereas the IR requires a CAA authorised Instrument Rating Examiner, who are more difficult to find and book.
And, of course, the initial IMC can also be done by a QFI, whereas the IR requires a visit to the CAA Flying Unit, who have a (rather undeserved) reputation for being dragons.
W