What Are The Benefits Of Switching From Prozac To Pristiq?

I am sorry to bother you. I have a questions. I have been taking 60 mg of prozac for about 4 years. This past week my Dr switched me to 50 mg of pristiq. I am not sure what the benefits of the switch are. Is the pristiq comparatively a smaller dose? The last time I took prozac 60 mg was Wednesday. Thursday i took my first dose of the pristiq. Is the pristiq supposed to be stonger or weaker than the prozac? I am just trying to keep an eye on how my body/mind was reacting to the switch.