Large bore flanges (above NPS 24") to ASME B16.47-Serie A or MSS-SP 44 are heavier/stronger than the ASME B16.47-Series B in the same size and pressure rating.
In most end-users piping specifications the Serie A is specified, which can whithstand more external loadings than the Serie B type.
For both series the dimensions are different. From a commercial point of view, the serie B type may be selected for piping with a non critical application.