RCA Coax or swap out the optical

I am running an Oppo BD 105 as a transport
unfortunately the Oppo only has optical and an rca coaxial digital out


the two dacs I would like to switch between 

Audio Research Dac 3 - not as detailed but oh so musical

Conventional 75 ohm coax and  Toslink optical inputs


MHDT Orchid - very detailed and tonally accurate

“BURT”. Digital inouts.   -  75 ohm BNC, usb, rca and.  toslink optical

since the Oppo only outputs optical and an rca digital

would I just be better off swapping out 
Digital optical cable between the 
Two 

or is there an rca optical solution that handles the 75 ohm coaxial connection correctly?
any nice sounding bang for the buck rca  digital cables?

thanks