i haven't seen ES 6 or 8 series anywhere yet. i've only seen the 5. i have a CS 8-2. the CS series are made out of a heavier duty acrylic (the thing feels bulletproof), while the ES is made out of a thinner, lighter acrylic.
the thinner, lighter acrylic is also what ASM skimmers are made of. so the only real difference between an
ASM skimmer and an ES is the neck (ER uses a threaded union, while ASM is a slip). the only benefit i can think of for the union neck is if you have low clearance. the union neck slides right off, while i assume you'll have to lift up the ASM cup to disconnect it from the main body.
i'm really happy with my CS8-2. but, if i was looking at an ES8-2, i'd definitely take a closer look at getting an ASM.