Chemically, Pristiq is almost exactly the same as Effexor.
For the most part, Pristiq was a way to extend the patent on Effexor, more or less calling it a different drug & charging more when it's nearly the same (desvenlafaxine, vs. venlafaxine) 1mg pristiq is roughly equal to 2mg effexor. There is a page discussing this on the pro section of psychcentral. Hopefully that puts you at some ease.