The main difference between the two streamers is that the Pioneer N-30, unlike the N-50, doesn't have the digital inputs to allow its internal DAC to be used as an external DAC by another device. It also lacks the N-50's more solid build, the extra power transformer (for the DAC section) and audio processing modes.
However, their DAC chips are actually the same and they certainly share the same network streaming, internet radio and USB storage input playback capabities. Of course this means the N-30 also shares the N-50's problems (where applicable), see my earlier post (#97) in this thread:
*Official* Pioneer N-50 Network Audio Player Thread
For network streaming its main problem is its lack of fast forward/back / slider positioning within a music track. Also gapless playback is not supported if you don't use the Pioneer ControlApp or its remote control / buttons on the box.
If you can put up with its short comings, $225 all in sounds like a good deal for a new decent network audio player, IMO. It should have no problems network streaming and playing back your hi-res FLAC & MP3 files stored in the USB drive that's connected to the iMac. I would, however, suggest using a different UPnP/DLNA server to the PS3 Media Server for it on the iMac. The PMS is a decent enough allrounder, but is mainly geared towards video, especially for its excellent transcoding capabilities and renderer/player profile setups. A more suitable UPnP media server to use would be the music dedicated (& free) MinimServer, available here:
You should be able to run PMS alongside MinimServer, if you are using PMS for something else, even if both applications have the same meda files linked into their media libraries.
Hope this all helps