I personally think the ISAF one is bloody awful anyway. The OSM is much nicer, heavier, shinier, and actually has your details on it.
I wouldn't really want to wear my ISAf one even if I was allowed. Although the little ribbon "badge" thing in the box is a damn good idea.
As someone else posted earlier, the ISAF medal fetches around £15 on ebay.
An OSM (proper one, beware copies) goes for about £300.
That says a lot.