After a ton of research on the Cyber Power website and side by side comparison, here are the answers to your question:
CP1500PFCLCD has the option of remote/auto shut down of your devices by buying and installing the RMCARD205 hardware to the UPS. This allows you to control or set up for auto shutdown of your devices. Go take a look at the pictures of the back of both UPS devices. The specs also state that the CP1500PFCLCD has SNMP communication but the GX1500u does not. Simple Network Management Protocol (SNMP) is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behavior. The CP1500PFCLCD as a clear obvious spot to install the RMCARD205, whereas the GX1500u does not. The product descriptions on Cyber Powers website clearly state that BTOH are PFC Pure Sine. I know it gets confusing when you actually search for True Sine UPS PFC and only the CP and OR models show up. However, as I do side by side-word for word comparison of the models, they both clearly state that they are AVR PFC True Sine UPS's. You are paying extra for remote access/auto controls. Also, the GX1500u only uses 1 type of model for battery replacement, whereas, the CP1500PFCLCD has 2 options for different model numbers.
Again, they are both True Sine PFC AVR UPS's.