I have them both here, and A/B'd yesterday.
On my rig, to my ears, I hear a bit more muddiness in the lows on the Senn, relative to the X2.
The X2, I cannot distinguish from a cable so far.
I do know that with by Boss LMB-3 engaged (Limiter pedal), on the EW172, I hear additional interference relative to a cord, but that's in our practice space. At home, I can't test if that goes away with the X2.
The build quality of the EW172 is orders of magnitude better than the X2.
The EW172 receiver and transmitter are nearly all metal construction, very rugged. It is also a little bit more complicated to set up.
The X2 receiver and transmitter are all plastic, with antennas like you find on a computer WiFi access point/hub. They're loose-ish at the pivot.
I like the antennas in the EW172 better. All metal, and nice tight solid feel where they pivot.
After only a few minutes with the X2 transmitter, it is already down one notch on the battery indicator.
With the EW172, I can play a 3-set gig on one set of AA's, and its only down 1 of 3 bars at the end of the evening.
So I suspect the EW172 transmitter will have longer battery life.
So, there are tradeoffs, depending on what is important to you. If it is all about purity of sound, I don't see how you can beat the X2.
But for high volume hard rock like what I play, does it really make a difference? I can't say.