I've owned a lot of these, so I'll give my personal opinions. I happen to love several.
The Grado sound is in-your-face, fun, detailed, and punchy. I think sound stage is small until you move into the 325's, which are my favorite (I have not heard the X version). They have a very focused and forward sound, in general, though, so it's a flavor that is divisive. It's easy to hear them and immediately think, "Yep, this is what I was looking for." I think my preference for music that is recorded poorly kept me from committing to the Grado brand.
The Sennheiser 580 or 600 headphones have a much more laid back presentation. Great detail, but with less airiness. They're comfortable and easy to listen to for a long time. I used 580's for over 10 years and upgraded to the 600's and had them for another 5. They were my choice for a long time. They take a little of the intensity and energy away, but the trade off is warmth and listenability. Very forgiving of poor recordings or amplification, IMO. In fact, they never sound bad. Haha. I have never wanted to turn them down.
I moved those along for the HiFiMan Sundara headphones. As my stereo got better and better, I wanted that kind of space, air, and detail in my headphones. Sundaras are, without question, the most like a pair of high fidelity speakers strapped to your head, but not without excellent room-like atmosphere. The soundstage is very articulate and instruments are easy to pinpoint. I have been living with these since the Sennheisers. They're a lot of fun, but I'm considering a change back to Sennheiser because I cannot help but feel like I spend more time listening to the headphones than the music. I have never been able to engage with my music like I did when I had either the 580's or the 600's. Having said that, I think if you're looking for the lowest price point that can bring you the highest fidelity and get a really good taste of what's up the stream, get Sundaras. I see them for $200 often, which is the best deal on hifi cans I've ever seen.
