There seems to be a dizzying array of KMC chains. I think the OP can only trust the bike shop here. In most cases you can identify the fancier X series by the fancy shape embossed on the outer plates.

I used a KMC 8.99 for a while. Worked well and looked nice. Didn't last that long compared to some other chains. It didn't 'stretch' particularly fast, but the rollers were all getting pitted after 3k miles or so, a little bit prematurely IMO. It was kind of strange. That's not an unreasonable lifespan I suppose, for a relatively inexpensive chain.