I'd be super extra careful in deciding between these two because they really couldn't be more different.
The biggest difference apart from how dark/bright they are is how they play and how they feel under the stick. This is a function of how thick and heavy they are.
If you're looking at pretty much any "Rock" line, know that they're going to be as heavy (for their size) as you'll find anywhere. That thickness, along with their high profile, raises their pitch high enough to clearly rise above walls of amps. But if you're not used to them, they will feel like manhole covers; your sticks will seem to bounce right off of them in a jarring way, and they won't be in any hurry to get out of the way. This is great in a loud rock band where you're swinging for the fences, but will be a major disappointment/limitation if you're looking for any kind of nuance.
K Customs, on the other hand, are pretty much on the opposite end of the spectrum since they typically break toward the thin end of the spectrum and lower profiles low the pitch. Because they're so much thinner, your stick will feel like it's getting absorbed right into the cymbal and it's not at all jarring to your hand. Sound-wise, they're also deeper and richer, and you get it all without having to bury the stick in it. If you do play in a loud rock band, you have to be careful not to overplay them because they do stop giving back after a certain point and they will crack (as even rock weight cymbals will do).
All branding and marketing aside, what you're essentially looking between these two are bright/heavy vs. dark/thin. But it isn't just that - you're looking at those in their extremes.
There are a lot of cymbals that occupy a more middle ground. The regular A Series are a good place to start, but again, pay close attention to the weights. A or K Series Zildjians make it simple because a medium-thin crash (for example) is called "Medium Thin Crash" (though K also adds the word "Dark").
You have to pay more attention to A Customs where the regular Crash is a thin; Projection Crash is medium thin; and Medium Crash is, well, medium. You get the idea. Same goes for K Customs. So you really have to educate yourself. Peruse the Zildjian website to see how they class the relative weights of their cymbals, and notice that Rock goes way beyond thin, past even just a medium, and lands way out there at Heavy, or even extra-heavy. They're not kidding, either; those are some heavy cymbals.
Armed with that information, get yourself to a music store and start checking them out and really pay attention to how they play. I'd also say that, independent of brand, the most versatility is with medium-thin weight crashes, medium rides, and medium over heavy hi hats.