PC (Linux) for the win. Can you even upgrade HDD space on macs without paying bananas for it? I'm not meaning external storage drives here.
But, every system (Macs, PCs / macos, linux, windows) has the same basic principles that a music library computer needs:
- Powerful organizer/player (iTunes, Amarok, Rhythmbox, Winamp, foobar, you name it)
- easy possibilities to backup the files (macs & linux can make automatical backups hourly/daily/weekly/monthly -- windows systems could use some external backup hdds with their easy-to-use one-button solutions)
- get out nice sound (digitally assuming. Macs have USB and SPDIF outs, PCs can have everything their builder wants).
So it's pretty much up to preference