USB-S/PDIF Converter or USB DAC?

I've decided it's time to use a music server with my main system. I currently use one (iBook) with my headphone system with the Grace m902 headphone amp with built-in USB DAC. I intend to get a new Intel iBook when they are released (March/April).

I currently have an unused Wavelength Audio S/PDIF DAC that I could use with a USB-S/PDIF converter (such as Empirical or Red Wine Audio) or get a USB DAC (WA Brick, Stello, ???). Is a dedicated USB DAC preferred over the hack USB-S/PDIF converter route?