Whoopies can slip,that's thier weak spot,loopies do not slip when choked with the bury section evenly split amongst the bight(or whatever you wanna call it)
Understand you get what you pay for most of the time,these are nice because they are cheap,way different breed than them sweet swivel mini blocks you mentioned,js
They are both merely spliced loops,he is referring to one as X(rigging prussic)& other as Y(locking prussic),they are different diameter loops but nothing specialy done to it for it to be dubbed 'the locking prussic '