You can use either, the following documentation might help you decide which type of layer would best fit your needs: http://help.arcgis.com/en/webapi/silverlight/apiref/ESRI.ArcGIS.Client~ESRI.ArcGIS.Client.Layer.html
http://help.arcgis.com/en/webapi/silverlight/help/index.html#/Feature_layer/016600000015000000/