Macmini is quieter (even after replacing the MDDs fans with ultraquiet ones), has lower power consumption (important when ~always-on), and a smaller footprint (sits next to the TV nicely without taking up half the room).
Downside is that the audio sucks, at least on mine (loads of interference from usb, graphics operations, network transfers). I still find it frustrating that we don't have isochronous transfers over USB just for this reason.
The VGA quality of mine sucks. DVI looks great.
Some of the G4 minis has infamously bad VGA quality. I remember being really disappointed when I bought it. I remember searching on forums and finding other people with similar complaints.
