If you ever spot Spam (either in the forums, or received via forum direct message) please use the Report button at the bottom of each post to make sure a Moderator can handle it quickly. Thanks for your help in keeping things running smoothly!
All other things being equal, an async USB link allows for better jitter management than an optical S/PDIF link.
Things however are rarely equal. It comes down to many factors, jitter mitigation in the DAC - perhaps it does a really good job with optical, galvanic linkage to the PC - maybe the PC has a noisy ground which gets exported down the USB link or perhaps the design of the DAC is poor in the first place .... and so on and so on.
All of these factors could favour one input over the other irrespective of the quality of the link or source of the digital data. My guess is with a decent DAC, you won't be able to tell the difference between your two proposed setups if you conducted a blind test.
The Chromecast is very cheap, and gives good results, why not buy it and try it ? USB cables are cheap too - do your own comparison.
I think the conventional wisdon is that a direct link via USB cable is the best, and that reflects the experience in my system. BUT Hi Fi Choice this month has an article 'Lets get digital' in which it says 'The USB option is simple and cheap....The streaming approach can potentially offer better quality'
Intrigueing ! They may be referring to Bluetooth Apt X (?)