My car is a 69 SS396/TH 400. What are the advantages of a high quality unit?

SOME of them will not come apart after years of service. Unless your racing it heavily I would stay with a stock unit.DO NOT BUY A FLUID TYPE (no name brands bashed) BALANCER, THEY ARE KNOWN TO BREAK CRANKS!!!!!

to answer your question a little more, as i understand it, a rattler or "fluid" type balancer will dynamically balance a range of vibration frequencies...a stock balancer is static as it doesnt change, but, theyre (stock) mostly fine for street/strip use but the elastomer (or whatever it is) filler is known to depart the assembly...theres a whole science behind vibrations and their frequencies eg: valve train componants...i'd like to hear more about these "crank breaking dampers"...any links, tech reports or articles? i know alot of builders are using rattlers over fluid but, ive never heard of a broken crank attributed to an undamaged damper