You might as well call them identical twins. Springfield designed their brand new 9mm XDs under the exact same skin as its widely popular 45 ACP brother, which was introduced in 2012. The only distinctions between the two carry guns are their magazine capacity, and the size of the hole at the end of their 3.3" barrels. The XDs just hit the shelves last week at a price of $599.
While they share identical ergnomics and sights, they also share inherently different recoil characteristics. Rob Leatham from Springfield Armory compares the performance between the 9mm and 45 Auto XDs: