EAI cogs appear to have the highest quality of machining, compared to some of the others.
Appearance doesn't necessarily mean anything.

That said, an article by Sheldon Brown or Phil Wood (can't remember) has me thinking a person should stick with one brand of cog, since the various cog manufacturers have tiny respective variations in their threading, and these variations, when one goes from one manufacturer to the other, will cause premature wear on the threads of the hub.

According to this thinking, if one goes back and forth between a DA cog and an EAI cog, each time he does so the new cog will rethread the hub to match the new cog's slightly different threading, and this will remove metal from the hub threads each time one installs the new cog.

Hm.

If so, then I say pick a manufacturer and stick with that manufacturer.

I like EAI for the appearance of EAI's machining, and for the broad range of teeth in both 3/32's and 1/8, and my current hub has never "seen" anything except an EAI cog.

As for chains, I started out with a KMC 610 and have since switched, following my lbs's recommendation, to a KMC 810.
I find the 810 quieter.

I have a separate chain for each chainring and cog combination.