Its not a fair comparo for the Focus RS to the Mustang GT, it will get made over and over again like beating a dead horse, but one is a hard hitting flyweight, the other is a middleweight that punches above.
As much as i love the 2015 Mustang (even the previous gen in fact), id much rather have the RS. More revs, easier to Tune (oh hai Cobb!), better MPG most likely, better for road trips and hauling around more than 2 people, better for adverse weather (im in Phoenix, dafuq am i talking about).
Its apples to potatos, but it will be put up against it often, and id love for it to be CLOSE.