Do you have an amp at the moment? If not, I would suggest you do buy one that supports at the very least LPCM via HDMI to enjoy the HD audio formats via bluray. The games are in Dolby Digital 5.1 at the moment and always will be AFAIK. Dolby Link. The HD audio formats will be used by bluray movies only.
As to which is better for games and SD movies; HDMI audio or optical? There is no difference.