Coax is better. I'm not an engineer and I'm not going to pretend so I can't back my answer up technically. If you were doing a ridiculously long run of cable, then optical might actually be better, but Coax is better.
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Coax is superior, if only because you don't have to convert an electrical signal to light and back. Many claim superior sound but I've never used Toslink so I cannot comment. Also, try to audition a nice high-end digital cable. I have the Illuminations D60 and was pleasantly surprised by how much of an upgrade it was for the price.
Coax is generally better for listening (e.g. connection from CDP to DAC to amp to speakers/headphones), but if your plan is to connect the CDP to the Marantz for purposes of recording CDs (????), then optical shouldn't be any worse if the bitstream is identical. Why buy a CDP with optical out only in the first place?