Though there are many types of full-size (circumaural) or earpad (supra-aural) headphones, for this blog I'm going to compare an open-back headphone from Grado, the SR225i ($200), with a closed-back headphone from Phiaton, the PS 500 ($299).
Sure, other manufacturers make open- and closed-back (aka sealed) headphones, but generalizations about the sound of the two types hold up pretty well. DJs, musicians, and recording engineers generally prefer closed headphones because they seal the wearer's ears, limiting how much sound they hear from the world around them, and at the same time, people close to the person wearing the headphones don't hear very much sound "leaking" from the headphones. So closed 'phones are great to wear in bed. Isolation from external sound isn't as effective as a noise-canceling headphone, but the closed-back headphone doesn't use batteries to power the noise-canceling circuitry. And closed-back headphones tend to make a lot more bass than similarly priced open-back designs.
The Phiaton PS 500's outer earcups and earpads are covered with genuine black leather, and the cloth-covered cable adds a touch of luxury to the design. It's a very comfortable and beautifully built headphone.
With an open-back headphone, like the Grado SR225i, you hear external sound quite clearly. This is a good thing if you ever want to listen on the street. Anyone near you will hear some of the sound of the Grado. Bass may not have the weight of a closed-back design, but the bass quality and definition are clearer than most closed-back designs. Open-back headphones tend to be directed to the audiophile market, but that's not to say there aren't closed models that appeal to audiophiles. For me, the biggest sonic difference is spatial: closed headphones make a sound that's "inside the head," and open models are literally more open, so they sound a bit more like speakers. The better closed headphones exhibit less of the inside-the-head quality, but they sound less open than the very best open headphone models.
So I hesitate to say one type is better than the other; it depends on what you're looking for from a headphone. I'm an audiophile so I prefer open headphones, but there are lots of folks who own both types and use them as needed.
My full CNET review of the Grado SR225i will be posted soon, but for now I'll say the Grado is a very detailed sounding headphone. Its open quality is hugely appealing, and it sounds "you-are-there" realistic with good quality recordings. It's not forgiving with harsh, dynamically compressed pop and rock recordings. Bass is nicely balanced, but a bit lightweight, compared with my much more expensive Grado RS-1, and most closed headphones.
The Phiaton PS 500 is a bass champ, the low-end really kicks! Definition is very good, but the Grado is better in that regard, but the Grado's bass feels lightweight compared to the Phiaton's. It's a classic closed vs. open conundrum.
From the midrange up it's a closer contest; that's what I thought while listening to Elton John's "Tumbleweed Connection" CD. John's vocals and the acoustic guitars were extremely present sounding over the SR225i; the PS 500 was darker toned and warmer sounding. Less immediate, and I couldn't hear the analog tape noise over the PS 500 as I did with the SR225i. Is that a good or bad thing? That depends, if you want to hear everything in the recording, it's bad; if you prefer a smoother, more relaxed sound that "hides" tape noise, then you'd prefer the PS 500.
These are two very different-sounding headphones, so I can't predict which one you'd like. It comes down to preferences and whether you need an open- or closed-back headphone. The best way to know for sure is to audition them side by side; if you can't do that, the first thing you have to do is decide if you need the isolation of a closed design, or the open quality of a Grado, Sennheiser, or another open model.