In practice , Theres very little difference between small boat EPIRBs and PLB's The main difference become more noticable for SOLAS speced EPRIBs or big ship EPIRBS. in practice a non-float free EPIRB and a PLB are very similar. I have a ACR PLB and a ACR EPIRB, eitehr is a good investment.
I'm not aware that any modern PLB use the 121Mhz freq, there all 406, in fact I dont think PLB's ever used it. The aircraft ones did but not PLBs. You comment is really valid in regards to modern PLB ( ie anything in th last couple of years)I guess a plb although less expense, especially after feb 2009, wont cut the mustard