You've hit the nail on the head. There is no real difference since both qualifications allow you to fly in cloud in the UK. The test standards are higher for the IR (painfully high, especially on the hold).
Perhaps most people would find 15 hours a little light in terms of shooting off into the clag on hour 16 on your own and if you want to fly to france the IR is useful .
If you were just flying privately in the UK then the IMC woudl save you a lot of money. If you want to fly commercailly the IR is the one to have. A lot of instructors have an IR but only renew every two years lapsing to IMC priviledges in the second year.