The following are my impressions. I am not a professional so take everything with a grain of salt.
I have in my possession both the X1 and X2.
The first thing I did after getting the X2 is comapre the two aesthetically.
The black on black looks really sleek. In terms of looks X2 wins this one.
I tested the swivel on each and the X1 has noticeably more. The cups can be moved around significantly more.
Observing the headband one will notice that the X2 has a higher peak. It is more "bent".
Looks: X2 wins
Putting them on is where you start feeling differences. The peak and the more limited swivel impact the comfort.
They "dig in" below my ears a little bit. Now I may be spoiled by the X1s, but these are not as comfortable. Period. No discussion about this one for me.
Comfort: X1 wins
Now for the interesting part; the sound.
Going forward I want to make a disclaimer: the differences aren't large, I am simply mentioning them because they exist.
Anyway, I did an A/B test on these.
Personally, I like to feel like I'm surrounded by the sound when I'm using headphones.
When I first got the X1, I was amazed by the depth. It felt like they could reproduce sounds to fill the room I was in.
When I tested the X2, this is where I found the largest difference between the two. The X1 made me feel like I was in a room, the X2 made it feel
like I was in the same room but without a roof. This is probably (imo) where the "darker" vs "brighter" arguments come from.
The X2 exposed the faults in the X1 that I hadn't realized before. To me, it just feels like the X2s are "airier". There seems a smidge more space between instruments.
The other aspect I've read about is the seeming lack of bass in the X2. In my opinion, this is not true. I still feel the kick without issue.
I think the reason people see the X2 as less bassy is because of the difference highlighted before (the closed room vs open roof room).
Purely soundwise, I'll have to hand it off to the X2, but BARELY.
For instrumental music, I'd go with X2. Drums feel fantastic. For music more along the lines of dubstep/hip-hop, I'd go with X1.
It feels like I'm beating a dead horse but I want to emphasize one more time that the differences are minimal (soundwise).
So what should you do?
I would go for the X2. My main reasons are the slightly more airier sound and the fact that they have replaceable pads.
I plan on keeping these cans for a LONG time, and replaceable pads were a must for holding on to these.
X1 is still a fantastic choice. It is a clear winner to me for comfort.
But make your own choice, this was just my two bits. Sorry if repeated myself too many times.
EDIT:
Saw a post about blind tests. Yes I could probably tell which ones were which.
Second, I ran both through a FiiO E7.