The silver amalgam ones include mercury, which is toxic, and once in the body, is hard to get out. There's a belief in some medical circles that mercury is gradually released from the fillings and can add significantly to the toxic load every modern person carries in their organs, bones, fat, and even breast milk. Because the most mercury is released when the fillings are new, I would avoid it for just that reason alone, even though those fillings will eventually fall out.
I became seriously ill with chemical sensitivities over 20 years ago, and my doctor convinced me to have ALL of my silver amalgam fillings replaced with gold. Each time I went for removal of one section of my teeth, I followed immediately with several hours of an intravenous chelating agent to remove whatever free mercury I had inhaled or swallowed during the dental procedure. It was a grueling summer, and horribly expensive (probably much more so now, with the price of gold at an all-time high), but it did seem to help my health overall.