Simoom is a strong, dry, dust-laden wind usually used to describe a local wind to blows in the Sahara, Palestine, Syria, Jordan and the Arabian Peninsula. It can exceed 54 degrees C and the humidity can drop bellow 10%.
Shamal is a northwesterly wind blowing over Iraq and Persian Golf states, very often strong during the day, but decreasing at night. This wind occurs anywhere from just once, to several times a year, and it creates large sandstorms.
The similarities between these two winds is that they are strong, dry winds, that cause sandstorms.