Strain gages are easy to mount, cheaper, and can be left in place for long term monitoring if they are properly protected. The selection would also depend on what parameters you are trying to measure. If your trying to determine the Dynamic load allowance factor or impact factor (increase in stresses) due to traffic then strain gages are way to go. If you trying to get the frequency and damping of the bridge, accelerometers are way to go, even though in some literature people have used strain gages. Like Tom Irvine said, you should be looking for "seismic" accelerometers.