CD20 is a pan B-cell marker and is one of the most utilized hematopathology stains.  The staining pattern morphology is membraneous and cytoplasmic.  The sensitivity and specificity for mature B-cell neoplasms is very high. 

CD19 is a B-cell marker.  Until recently this antibody has only been available for use in flow cytometry.  Now CD19 is available as an immunohistochemistry (IHC) marker, with similar sensitivity for B-cells to CD20.  The IHC expression pattern is cytoplasmic and membranous.  It is normally expressed on both Precursor B-cells and B-cells.  CD19 combined with TdT would be specific to detect precursor B-cells. CD20 is not expressed on precursor B-cells, but mature B-cell with surface light chain expression (usually).