[Tofujr]

I have an Asus N56 laptop with no dedicated sound card, I believe. It supports a SPDIF output labelled in windows as Realtek Digital Output. This is running to a Little Dot Dac_1. 
 
I can't tell a difference between 16bit/44.1k USB out and 24bit/44.1k SPDIF/Toslink out (The dac doesn't support 96k and 192k I think). 
 
However I'm a firm believer of...placebo. So uh...which one is better?  

 

[ToddTheMetalGod]

I would use USB if I were you, that way you can bypass the sound drivers and processing in the on-board sound which can mess with the bits and add slight changes to the sound. With my on-board sound I need to use an ASIO driver with bit-perfect output or else the sound seems to ever so slightly lose clarity and detail. I believe it's because of the Realtek driver mixing the sound, but it doesn't sound quite as natural. Passing the sound through USB should make no audible difference, but some people are convinced in buying hi-fi USB cables which literally make no difference in audio quality since it's a data cable (unless your current cable is terrible that is).