The IMC rating, is basically a "instrument rating-light" there is a little less you have to do for it, and the hour requirement is less. In return among other things, the minimums are higher.
It is a UK rating, and only valid in the UK
The IMC rating is not an instrument rating and, originally intended for getting people out of trouble when they get stuck, should not be used as such.
If you are planning on doing your instrument rating, getting the IMC rating first won't give you an advantage really.