Both keyboards share the same overall design aesthetic and feature Logitech's new Romer GL switches. According to Logitech, these low-profile switches are roughly half the height of normal mechanical switches and have 25 percent faster actuation. Enthusiasts can choose from GL Linear, GL Tactile, or GL Clicky switches depending on your feedback preference.
Getting down to the differences between the two keyboards, the G815 Lightsync RGB is the wired version and includes five programmable G-keys and supports three onboard profiles. As its name implies, it supports Logitech's Lightsync RGB technology and there is a USB 2.0 passthrough so you don't have to "give up" a USB port when plugging in your keyboard.
The G915 Lightspeed is the higher-end, wireless variant of the keyboard. Built around Logitech's Lightspeed wireless technology, this keyboard is said to provide over four months of battery life if you have the RGB lighting effects turned off. That figure is based on using the keyboard for 8 hours a day. On the other hand, if you have the RGB lighting effects enabled, the keyboard will last just 12 hours per charge. Given that Lightspeed is using Logitech's proprietary wireless technology, you will have to use a USB dongle to plug into your PC instead of relying on the Bluetooth standard.
Logitech says that both the G815 Lightsync RGB and the G915 Lightspeed will be available later this month from your favorite retailers for $199.99 and $249.99 respectively. If you prefer, you will also be able to purchase the keyboards direct from LogitechG.com.