For a data server I would prefer something I can load up with hard drives and use RAID 5 or better. Though some other OSes could be more of a hassle to setup initially. Once it is setup it really doesn't matter if it is running Linux, Windows or OS X. If it is just going to sit in a closet and serve data.
Plus in the future have the option of putting a PCI-e x8 RAID card to handle numerous uber fast high capacity SSD's and a PCI-e x8 Intel Lightpeak card in a few years for a moderately priced 10gb fibre optic network. Just to be able to stream full blu-ray rips to every HD tv in the house.
If you just need it to serve a bunch of music, documents and pics you could save a fortune with a used PowerMac G4 and load it with hard drives.