For about $550 I can get the NUC box with a core i5, 8GB of RAM, 240GB SSD drive, no OS. For $50 more I can get the low end Mac Mini, it has less ram but more disk space (not SSD though).
For that trade-off in price I would still pick the Mac Mini, it'll have a nice warranty, come with an OS I won't have to screw with, and I can upgrade the RAM later for not much money.