Both are digital of course and some people say that there's no audible difference between the two. TOSLink however is supposed to suffer from more jitter problems. Google it up, you'll find a lot of forum threads where people ask the same thing.
There is no “better” interface. Each has their advantages and disadvantages (and optimal application). TOSLINK is isolated electrically while coaxial isn’t, though it usually has less jitter inherently. However, both are bandwidth limited (compared to AES/XLR) and both have optimal lengths. Glass TOSLINK is usually preferable to coaxial in computer environments, though properly-terminated 75-ohm coaxial digital (or as close to this as possible) is probably just as good. Some soundcards have better SPDIF outputs than others and others have poorly implemented TOSLINK or coaxial. Some DACs perform better with one or the other while others are interface-immune (or at least interface difference-resistant). I prefer AES/XLR to either format, but most soundcards don’t have AES/XLR output (even the E-MU’s AES/XLR output isn’t fully realized). Therefore I use the RME Digi 96/8 PAD for its superior AES/XLR digital output for my music, and the E-MU’s TOSLINK output for gaming/windows (the RME doesn’t support DirectSound). I also use the coaxial digital interface with my Panasonic DVD-S47 DVD-Audio player because I’m not worried about electrical isolation with that configuration and because the TOSLINK output on the DVD-S47 is limited to 96kHz while the coaxial interface allows full 192kHz output. Thankfully the DAC1 supports all three interfaces. Soon a quality, jitter-resistant DAC will emerge that can accept a firewire signal – that is the ultimate interface, IMO.