The display and much of the software is the same, as are the controls, syntax, etc. The big difference is that the G3X gives you an AHRS, so you get attitude and air data information - a real PFD. The 696 is "just" a GPS...with databases....and Geo-referenced charts....and XM.....
Thanks, Paul...I'm only looking for moving map, anyway, but I've been holding our for a few features that not all systems have. With georeffed approach plates, the 696 is getting close.
Paul is correct that the G3X adds the ADAHRS to the display system. With this addition you get the PFD and MFD screens whether you use 1, 2, or 3 screens. The G3X can also be interfaced with engine sensors to display engine parameters on the screen as well.
The GDU 375 (which can be installed alone in the panel) is very similar to the GPSMAP 696 with the differences being the XM receiver is internal and database updating is through a front SD card slot. The GDU 375 also has a connector on the back to wire it in the aircraft for power and ground (has redundant power inputs). Additionally it has multiple serial ports for several interfaces.
The GDU 375 will support the same traffic systems the GPSMAP 696 will. These will use the same input and settings as the GTX 330 uses over the RS 232 bus.