Based on my extensive knowledge of watching Breaking Bad, I'd say the answer is purity.

“Biker meth” is most if not all meth now. It uses phenyl-2-propanone as the starting point and produces a racemic mixture of dextromethamphetamine and levomethamphetamine.

Traditional clandestine meth production using ephedrine produces pure dextromethamphetamine.

d-methamphetamine is the euphoric feel good stuff. l-methamphetamine is the racing heart, more neurotoxic, nearly immediate psychosis stuff.

TL;DR - “Biker meth” is most meth now. It’s much worse for you and less enjoyable.