I do not know what differences the current versions may have. However, the RR501 had a stronger PLC output (10Vpp vs. 5Vpp) than did any of the PLC transmitters designed/redesigned after introduction of the CM11A. One with a 2011 datecode still had the higher output while a redesigned TM751 from 2011 had the lower output.
https://www.laser.com/dhouston/x10-sig.htmlAlso, the RR501 had an RF receiver that was integrated into the main circuit board so there was, perhaps, less reason to redesign it.
But I do not know whether the current RR501 still has that higher output nor do I know what the PLC output level of the PAT03 might be.
Were I in the market for one or the other, I would first ask X10.com about the PLC output levels of each before deciding.