The ADA recognizes amalgams fillings as legitimate restoration. There is literature that says the white filling is just as strong as the silver, but for most clinically it does not appear that way. That is in the real world the silver filling last longer and cost about 50% less. Silver fillings have been around since the start of modern dentistry. The physical properties of amalgam is similar to that of enamel except color, eg they both expand a simlilar rates when heat is present. Amalgam is more technique friendly, where as white fillings ar more technique sensitive ( meaning that more things can go wrong, eg if a dry field is not maintained and saliva gets into the area and not noticed the filling can fail)
As far as the literature is concern, the white filling may be as strong as a silver filling, but those studies fail to mention that white fillings are held in by an adhesive whch tends to fail in 2 to five years. Once the adhesice fail the bond is bronken and the filling is vulnerable to failure.
My opinion is the the silver filling is superior to the white composite filling. In myclinical experience its not uncommon to find silver fillings that have been in the mouth for over 30 years, plus they are less expensive.
As for mercury, the way amalgam is formulated the reaction in the capsule goes to completion with the mercury being the limiting reagant and the other material being in abundance. Once the reaction is complete the amalgam is no longer mercury, and does not have the properties of mercury. think of it as water H2O while water has both oxygen and hydrogen, once it has become water it loses it properties of hydrogen and oxgygen, thats why you can't breath h2o.
White fillings are full of chemicals and acids are used to bond them