SPDIF vs. Optical: Which converter is better for 3.5mm analog output?

So, here's my current situation: I'm working with a motherboard that features both an SPDIF port and an Optical audio output. From what I understand, these are digital signal outputs designed to eventually be converted into an analog signal so they can actually drive my speakers or headphones.

What I'm really wrestling with is whether it would actually be worth my while to pick up some sort of DAC converter from eBay—one that handles that digital-to-analog conversion—just so I can plug in my 3.5mm headphones or speakers. My main concern, though, is the actual sound quality.

I'm trying to figure out if there will be any noticeable difference in audio fidelity between using the onboard audio processing that's already built into the board (via those standard 3.5mm jacks) versus buying an external converter $33 and then plugging my gear into that instead.

For instance, I was looking at this specific converter

http://www.ebay.com/itm/Digital-Opti...3D230675341215

My primary goal is just to enjoy higher-quality music and clear dialogue during video tutorials, and then, secondary to that, some gaming—though I usually stick to stereo since I simply don't have the floor space in my room for a full 5.1 surround setup like the Logitech x530 system I ended up with after getting a deal through AT&T. I also have these slightly nicer General Electric headphones from $67 which are actually quite pleasant to listen to.

Well, you might get a bit more punch out of the audio if you run it through a decent SPDIF or RCA converter rather than just plugging straight into that 3.5mm headphone jack. It’s not some ironclad rule, though... so...
Since those converters aren't exactly breaking the bank, there's really no harm in picking one up.
But honestly, it usually comes down to that old saying: you get what you pay for.
...plus, once you've got those RCA L/R outputs, you can't just plug headphones directly into the converter; you'd need to run them into an amp, a receiver, or maybe some powered speakers that have analog inputs.

At the end of the day, the sound quality is mostly going to depend on how good the sound card inside your PC or laptop actually is.
Of course, if you really want the gold standard, the best way to go would be a solid external DAC—ideally one with a dedicated headphone preamp built in.

moved over from Sony 😉