Whats the difference?
In a nutshell dyeability. Nylon plastic sliders you should in theory be able to easily dye them with RIT or acid dyes. They are often a dingier color of white so not the best choice if you’re making a white bra. Polyester sliders are using a nice bright optic white but alas not dyeable unless your using a professional grade disperse dye and even then your results are not guaranteed. They will usually stain a pale pastel color with acid or rit though.
Right now I will be listing the polyester kind. I have a few of the nylon that dyes really well but they are hard to source. Sometimes when you’re ordering wholesale you don’t know whether something will be dyeable until you test it. I test all my dyeable supplies so I can be confident in what I am selling.