The database for the Avidyne is a little more expensive than the Garmin GTN series. Jepp charges about $450/year and Garmin charges $300/year for a NavData only subscription, which is all that you need. As was mentioned, the 60-day free trial gives you terrain, obstacles, basemap and Charts as well as terrain. With the Jepp subscription for Avidyne you also get to put it on 2 iPads to run the IFD100 App, so the difference, in my opinion, is worth it.
The Avidyne comes with Wifi/Bt as well, including a bluetooth keyboard.
As for compatibility, the Avidyne will talk to a G5/GFC500 system just as well as a GTN will. The ARINC format is standard as is the MapMX format of RS232, which is how the Avidyne and GTN will talk. Some Garmin hardware plays nicer with other Garmin hardware, but for the GNS and GTN navigators, they play with everything as well as possible. The Avidyne actually has some data formats with labels the Garmin doesn't have. I have a IFD540 in my 182 with a G5 HSI. I used to have a GTN650 driving the HSI. There is absolutely no difference between the two in compatibility. The GFC500 talks to the G5, not the GTN, so it doesn't know and doesn't care if is has a GNS, GTN or IFD.