I also have a mac mini and am using the Logitech C920. At the beggining, I had issues with its built in mics not working with FaceTime, but I found a solution for the problem and now all the programs that interface with the camera work great (FT, Skype, iMovie, etc). If you do buy the C920 run into the same issue that I had with the mic not working, here is the quick solution for the problem:
Cause of Problem: The Logitech C920 camera/mic operates on a 2ch-16bit integer. You can verify this by opening "finder + Utilities + Audio MIDI Setup" and then select the HD Pro Webcam C920 and look at the "Input" Default values.
The Mac Mini's "Built-In Output" and "Buiilt-In Input" formats were set for 2ch-24 bit integer. Therefore, the default input and output were out of sync with the C920 default format.
Solution: Change the Mac Mini's default Output to "2ch-16 bit integer". You don't need to change the default Input but it doesn't hurt to do so. Once you do this, the camera and it's built in mics work on facetime and all other video chat applications.
i just figured the problem and has nothing to do with changing those integers. in the audio MIDI setting for the c920 make sure the format is set to 24000hz, and it will finally work. if its set to 16000hz or 32000hz it wont work. i actually went backand put the integer settings to its original settings. 24integer for mac and 16 for c920.