Calyrex-Shadow has more speed so that makes it fast, Calyrex-Ice has more def. and sp.def. so that makes it bulky.
Calyrex-Shadow has two weakness but they are 4x weaknesses so they are absolutely painful. Calyrex-Ice has a lot more weaknesses but they all are 2x weaknesses.
You can get calyrex-shadow if you want to hit fast and strong, but if the foe doesn't die, you can be lost. If you ask me, I choose Calyrex-Ice, because I can endure hits and then attack to them.