Composites if placed with exacting technique (rubber dam isolation and modern bonding agents) get as close as we have ever been able to restore the tooth to its original strength in the most conservative fashion. The composites are a very hard "plastic" that bonds (micromechanically fuse) to the tooth to hold the tooth together. Amalgams have multiple disadvantages.
Amalgam is a metal which expands and contracts with hot, cold and biting. Therefore, with every meal the teeth are being weakened and over time and depending on the size of the filling the tooth becomes susceptible to fracturing. Amalgam also contains mercury which is a very toxic element. The literature is very unclear as to the amount of mercury that is released from amalgams and the long term effects on the entire body.