HDMI vs Optical
HDMI and optical are the two ports that can carry high quality audio from one device to another. There are major differences between these two though that you should consider before opting for one or the other. Optical link, or more commonly known as TOSLINK, was intended to carry audio only while HDMI was intended to carry HD quality video, multiple audio streams, and even a CEC channel. For this reason, if you need to connect for audio only, then optical may be good enough. But if you need to transport more than audio, you may be better off with HDMI.
These two also have differences in how their cables are constructed. HDMI uses the more traditional material, copper, which is cheap but can be affected by interference. On the other hand, optical uses a fiber optic strand that carries light instead of electricity. Although it might be a bit more expensive per meter, it is totally impervious to external interference as no other light would penetrate its shielding. HDMI also uses 19 separate links to carry all of the signals that it can accommodate. While it is possible to use more than one fiber optic cable, the TOSLINK design only calls for one.
In terms of the maximum length for both cables, optical has a technical maximum of 10m but a lot users have installed up to 30m of uncut cable even when using cheap parts. For HDMI, there is not specified maximum cable length that you can use but the bandwidth that HDMI can use gradually decreases as the cable becomes longer. For applications that require high speeds, 5M is the typical maximum length while up to 15m cable can be used for standard uses.
Even if you only intend to use the cable for transporting audio, there is still a reason to prefer HDMI over an optical link. Optical does not support the Dolby Digital Plus, TrueHD, and DTS HD formats, which are supported under HDMI. If your devices are not that far apart and supports HDMI, it is certainly the safer choice between the two.
Summary:
1. HDMI is intended to carry both video and audio while optical only carries audio
2. HDMI uses copper cables while optical uses a fiber optic cable
3. HDMI uses up to 19 discrete links while optical only uses one
4. HDMI can be used up to 15m while optical is regularly used for up to 30m
5. HDMI supports more digital audio formats than optical