Should I use RCA cables or Digital cables for interconnects

I am getting a Marantz SA-KI arriving soon and I was wondering what would be the best way to connect the Ruby to the AV8805 preamp. I have a pair of purist audio RCA cables, An Audioquest carbon Optilink and an Audioquest forest Coax. I was told on my oppo 105d I could bypass the oppo DAC using digital and use the DAC in the preamp. I am not sure why but not using the RCA for the interconnect seemed more pleasing so I switched. My questions are 1) Does using digital bypass the DAC? 2) Which would be the preferable route with the Ruby?
Thank you,
Using digital out bypasses the DAC. If you use digital out on the Ruby and use the DAC on the 8805 it won't play the SACD layer on the CD, you would need to use the RCA analog output for SACD.
Be aware that if you use any sort of DSP or subwoofer crossover setttings in the AV8805, the AV8805 will automatically convert any analog inputs to digital to process the DSP/Audessey/crossover and then re-route it to the internal AV8805 DAC anyways.  I'm not sure if there is a way to configure the AV8805 to completely bypass any internal DSP or DAC. 

You would want a complete analog passthrough (maybe PureDirect, but even PureDirect will use the subwoofer crossover).  The SA-KI is such an expensive and nice unit, it's almost a mismatch pairing with the AV8805. 
I took possession of Ruby today. The dealer suggested I hookup both digitally and RCA and play with it. I used a pair of Purist Audio RCA cables and I can't quit listening. Thanks for the help and info, big help.
This is a reply from Marantz Support. I thought I would pass along:
Hello Santsuru,

Thank you for contacting Marantz Support, we value you as a customer and appreciate the opportunity to be of assistance!
If you use a digital cable, the signal will have to be processed a second time by thee av8805 DAC. Given the quality of the DAC in the av8805 there is not likely to be a discernable difference in most cases. However in general, you would want a signal to only be processed by one DAC, so the better option would be to use an analog connection between the units, and have the av8805 in direct mode.
Marantz does not have an official position on cables, but i can off you my own opinion and make of it what you will. Poorly made cables can and will degrade the quality of audio.Well made cables will prevent a loss of signal, but cannot improve it. As a result, once you get a cable that is "good enough" anything beyond that point will not make a difference. Price can be suggestive of build quality, but isnt always. There are some cheap cables that will work well, and there are many brands that spend more money on marketing their expensive cables then actually making them. I personally prefer to look for moderately priced cables from respected brands as those frequently fall into the "good enough" group.
If you have any further questions please feel free to contact us again.

Thank you for contacting Sound United Customer Support. Have a great day!
The Ruby has one of the best DACs ever built into a CD player. You should pair it with a much better integrated amp or a preamp/amp combination to unveil its sound potential. A Ruby is an affordable sibling of the 10 flagship series by Marantz. So don't degrade it by using it in an average system. It deserves the best partners you can afford, for example the dedicated Ruby amp. Loudspeakers? High solution preferred because those Ruby machines are hires specialists...

Greetz, Hans
Thank you for your information. I using a Marantz AV8805 preamp and a Sunfire TGA-7401 combined with B&W 805 D3's, a B&W 600 series center channel, with two CM5's for the surround speakers. I took the advice I was given and tried both digital connects as well as RCA connects. Yes everyone is correct the Ruby has excellent DAC and I am using Nordost RCA connects.