Generally there is no difference. Digital data is digital data...
An optical S/PDIF connection avoids ground loops which can happen with an RCA connection. A ground loop can result in hum, but that's hum getting-into the analog circuitry.
Wireless connections also minimize the possibility of ground loops, but there's more chance of "glitching" (when the transmission gets interrupted or interfered with). You are also more likely to get latency (delay) but a few milliseconds of delay is only a problem if you are watching a video and the audio & video are out-of-sync.
I
think USB can handle more formats because any conversion can be done with the software/drivers. For example, most stand-alone DACs can't decode Dolby Digital or any of the other compressed audio from DVDs & Blu-Rays but the USB should work just like any regular soundcard. But for the same reason, it might sometimes "hide" conversions or down-sampling.
Is it true RCA ports are weaker?
I'm not sure what that means with digital.... If the digital signal gets too week so the 1's & 0's can't be distinguished you've got "big problems". Digital doesn't usually degrade gradually or subtly like analog... You won't get a loss of volume or a loss of bass, etc., like can happen in analog. Usually It's clicks & pops, or dropouts, or it just doesn't work at all. My usual analogy is - One wrong bit in your bank account is just as likely to cause a 1 billion dollar error as a 1-cent error. (That's not exactly "fair" because one huge error in audio can fly-by, unnoticed and unheard, in microseconds.)
And you just want to listen to music on macbook or PC.
Sometimes the DAC built-into your soundcard/soundchip is good enough.
I think Macs have a pretty good reputation with PCs varying more, and sometimes you'll get noise. Usually noise is the only "sound quality" issue so if you're not hearing background nose a separate DAC is unlikely to hear an improvement.
Or, if you don't get enough power/loudness into your headphones, a DAC will usually have a better headphone output, or you might just need a headphone amp.