Corsair's K70 RGB doesn't fare quite as well in backlight design as the BlackWidow Chroma, but with the customization software, you have access to more options and possibilities. Of course, that software is more difficult to use due to its complexity. The RGB Cherry MX switches have built-in ESD protection and the good name of Cherry behind them. Corsair makes the best gaming keyboards you can get (if you want macro keys, go for the K95).
Overall, I'd pick the Corsair Vengeance K70 RGB due to the fact that you get more control over your keyboard, even if it takes more time and energy. I like having more options available to me. The Razer BlackWidow Chroma is a great gaming keyboard, but the K70 edges it out by just a hair.