I have both right now (I am planning on returning/selling one). With only ~$100 separating them, it's a tight race, and comes down to what you value.
I needed a closed-back headphone for office listening; I listen to alternative, some electronic, indie stuff, and instrumental pieces occasionally. I wanted to stay around $350, so DT 700 Pro X ($300) was my under and Elegia ($400) was my over. Sound isolation and long term listening are both important to me.
My ideal sound: I have the Avantone Planar (v1, open back), paired with the Qudelix 5k and Oratory1990's EQ. I've never wanted better sounds than this/couldn't tell you what they would be; the perfect closed back would just be this packaged up together.
DT 700 Pro X sounds good without EQ, but has that narrow/muffled sort of sound that people associate with "diminished" or even poor sound staging. Sometimes this translates to slightly shallow sounding background parts. It's built like a tank though and holds onto your head like life depends on it.
Elegia fits like a cloud on my head, and the ear cups fit over my ears comfortably (larger than DT Pro 700 X). I find them too bright at stock tuning, with Oratory1990 EQ however they are more "musical" sounding (warm is probably what people call it). There is a fullness to the sound that makes it immersive, and more rich than the DT Pro X. There are some QA problems to be expected with the headband; I am prepared to make an Eldridge abomination using Hifiman replacement parts when that bridge arrives (thanks to the people who made this nightmare fuel possible).
On detail retrieval, there are a few times I have caught the DTs being slightly less clear/having less separation on fast moving or busy instrumentation. The Elegia seems to balance the details better and doesn't lose them. Example, on a guitar riff, the DTs make the first notes energetic and well impacted, but then can trail off slightly muddily. The Elegia keep presentation clean and even, but can be sometimes boring to listen to without the impact variation (might also be the more balanced bass focus, lol). It's really only a slight difference, you probably wouldn't notice this during normal listening conditions.
Sound: Without EQ, the 700 Pro X take it for me, but with EQ, it's the Elegia. That wider and richer sound is closer to what my Avantone's make. Someone mentioned that there is a sort of tonality or metallic air to the Elegia; that could be true, but it's not very obvious or apparent with the set-up I am using right now. It could have been absorbed in getting adjusted to these headphones. Either way, it is never empty or tinny sounding with the Elegia.
For the Elegia on bass: when I am listening to a bass centric song, the bass is there. With the 700 Pro X, bass is slightly (but noticeably) more present otherwise.
Isolation: The DT Pro X is more isolating, but the Focal is plenty for me as well, even at very quiet volumes, enough so that I am oblivious of surroundings to a fault sometimes.
Build: Who doesn't want a gray and black sleek tank? DT takes it for aesthetics. The Elegia aren't quite as modern looking imo, but they aren't ugly either. The single jack on the DT is appealing.
Comfort: Elegia by a mile. As mentioned before, the ear cups are bigger. I think they disperse weight a bit better as well. Can wear these for hours without issue. DT Pro X isn't bad, especially considering that you can bend the metal headband into shape, but the Elegia is a master here.
Comparing to my Avantone Planar… Neither closed back has the same level of sound stage, but the Elegia gets much closer than the 700 Pro X with my EQ attempts. Detail retrieval isn't different in any obvious ways (Avantone still has the top). The Elegia can have fatiguing/harsh impacts for repeating tones in the mid to upper ranges and seems to impact some tones harder than my Avantone presents, but generally connects sounds together very well. The 700 Pro X can lose clarity when parts get busy despite having separation on par with Elegia, and feels farther away from what the Avantone delivers.
TL;DR: In looking for a closed back headphone for office use that sounds similar to V shaped open back headphones, I found the Elegia to be a viable option due to good sound staging, EQ friendliness, and general musicality. The DT 700 Pro X is a valid contender on a technical level, but doesn't capture the same voice/tone in music that I enjoy.