The Legato has a coax SPDIF output. It is a true 75 ohm output, transformer-coupled, via a BNC connector. Since most of you who have a DAC are stuck with the inadequate RCA jacks, we provide a cable and a BNC-RCA adapter. While this is not the optimum configuration, it will perform better than a cable with RCAs on both ends.
The differences between coax and AES/EBU are not much. Impedance, connector type, signal level are slightly different. TOSLINK is an optical interface, that has some troubling problems. Yes, it provides galvanic isolation, but has other problems, that make it the least desirable of the group.
To be honest, we are not experts in computer audio. Our expertise is in digital transmission. The only advice I can honestly give is to save your money, and buy a Mac. (No, no one here can afford one. So much for getting rich in audio!) It has been our experience, as well as lots of our customers, that PCs yield highly variable results, while the Macs always work great.
I would advise you to look upon the computer as the transport. Use it for a music server only, not something to play games and surf the Web, at the same time.
Even if you do buy a Mac, get the most memory, biggest HD and fastest CPU you can afford. The guy who writes the code says "You need enough horsepower to overwhelm the problems in the s/w." As you can imagine, PCs need lots of horsepower, to begin to sound good.
That is the extent of what I can tell you about computer audio.
Pat