This is an age old question that comes up a lot and many people have many different answers and opinions to this question. There are many things to consider when answering which filling material is better. Most of the research has shown that silver fillings last longer on average (only by a couple of years, 15 vs 17). The preparation that goes into a silver filling usually involves taking out more tooth structure, then you would for a tooth colored filling (silver fillings will usually last longer in bigger fillings because the tooth structure is already missing).
Another thing to consider is where the filling is located. If it is on the back of your last tooth on the top of your mouth, the esthetics of having a silver filling is not nearly as bad as having it on the front of your front tooth. Another variable that is important to some is the type of material that it is makes up the filling. It has been long established that silver fillings contain mercury. For the most part this mercury is stable inside the filling (it is worse for the dentist who is taking out a silver filling because that person is vaporizing the mercury and breathing it in everyday). This may seem bad but then we have to remember tooth colored filling material usually contains BPA (the long term effects of this are still unknown). Most people still prefer the white color filling because of its esthetics and the fact that less of your tooth has to be removed.
Hope this helps.
Best of luck to you!
William F. Scott IV, DMD