If you aren't planning on getting a Blu-ray player, or a DVD player that has DVD-Audio*1 playback capabilities over HDMI then you wouldn't be at an immediate advantage to use HDMI for your audio. Coaxial will handle 2 channel PCM, Dolby Digital, and DTS sound just as well as HDMI. However for Blu-ray you would probably benefit from the use of the HDMI connection as it handles HD audio that Blu-ray provides.
*1DVD-Audio was a competing audio only format with SACD, not to be confused with regular sound tracks found on all regular DVDs).