Multi-Channel, Digital Coax, or Optical?!!

[Col. Zombie]

I have been lurking here for a while now and while I know that similar topics surrounding my particular question have probaly been beat to death in a dozen different ways, I'm still at a loss. I tried the search function and really couldn't find anything that's as pointed as the questions I have. If anyone could help me I would really appreciate it.

I currently have my blu-ray player hooked up to my Onkyo receiver via the optical cable. My Onkyo does not have an HDMI input, but only the 5.1 Multi-channel, Digital Coax, and Optical inputs. My blu-ray player also has all three of those outputs and it has a built in Dolby Digital True-HD decoder.

My questions are;

Which of those three will give me the better sound quality?

Will Dolby Digital True HD sound better through the Multi-channel, or am I better off just running optical or coax using the standard DD and DTS? (I ask this becasue I haven't tried to hook it up using Multi-Chanel yet and I'm wondering if it will be worth my time).

Does the Multi-channel connection give the "Uncompressed 5.1" surround sound?

Thanks for your help with this!!