From my personal experience a dedicated streamer "sounds" better than a standard computer (although I've not tried a Mac Mini yet) - however - I've never bothered to soup up and fine tune a computer to the nth degree (take a look on the JPlay website to see how far you can go!).
A solid streamer like a Yamaha CD-N500 or Marantz NA 7004 already have the good quality power supplies, no moving parts, SQ optimisations, decent S/PDIF connectivity, so you don't need to bother faffing around with upgrades - and you have dedicated software for playing the music without sharing it's resources to compete with IO operations elsewhere.
Pretty spot-on summary.
The trouble with SPDIF is that isn't USB.
The trouble with USB is that it's not universal.
There's no doubt that an audio-developed PC is the top rung of the ladder, sonically, but it still 'feels wrong' to buy one for many - and it's not quite as plug and play, although - in reality - once set up, it's just as appliance-like.
I've been really impressed by devices like the Lumin recently: the future looks more Lumin-shaped to me than anything recognisably involving a PC.