Good question and there are a couple answers.
Really it all depends on what you're watching. Like I said before in my thread, if you are watching anything on your PS3 for example, that is a Blu-ray player as well, then you want to use HDMI for your audio.
The reason I say this is because it has the ability to carry these new audio formats. They are really big in size and need to be transfered down a cable that can handle the bandwidth. Toslink is not capable of carrying those signals.
All this other stuff also depends on how well you receiver is you have, your speakers, and the source you are using for the audio. If the receiver doesn't support those formats, then all you need is toslink. If you don't have a high end audio system, then you probably wouldn't notice a difference anyways.
Look at the back of your DVDs from now on and look at what the audio is encoded with. It should say something like Dolby Digital, DTS, Dolby ProLogic, etc..
Most Blu-ray movies will include a HD audio format of some sort. Some will not carry them at all.
HDMI = Dolby ProLogic,Dolby Digital, Dolby Digital Plus, Dolby TrueHD, and DTS-HD
Toslink = Dolby ProLogic, Dolby Digital, and DTS
So if you are using toslink right now and your receiver doesn't support the new audio formats, then you are probably safe to stay hooked up that way.
High-Definition Multimedia Interface - Wikipedia, the free encyclopedia
TOSLINK - Wikipedia, the free encyclopedia
BTW.. WIPE YOUR DAMN STUFF UP!