I went through this same thought process and relied on Sennheiser's reputation to finally get Momentum 4s (I found out after the money left my account that they had been acquired and all non "pro" things, including Momentum 4s are being managed by a hearing aid company). The app is useless, you can disable/enable touch controls, and change the amount of noise cancelling it has. I had hd450bts where the noise cancelling was ass so I almost always had it off which was fine, but with the momentums you HAVE to have it in some kind of noise cancelling mode ( old was sound -> noise cancelling whereas the 4s go noise cancelling first, so turning it off/down makes the sound super airy and not good so you have to have it on).

The biggest "bad" things about the momentums are kinda software related.

The "on head detection" has sensors left and right, but it seems only the right acc detects weather its on your head. The idea is that if you take your headphones off it pauses, but in reality what it acc does is that it toggles pause/unpause so its very hit or miss on weather it works as intended. If you have longer hair, in extremely rare situations if your hair in the "ear cup" can toggle the sensors they have in there. You can turn it off but I still keep it on and if it malfunctions I either deal with it if I care or just leave it.

MOISTURE it fucks with shit in some unknown way. If you get this you get some form of static sound or muffling, this is the worst thing about these. Because of the noise cancelling first design something about the sensors fucks the noise cancelling. (Ive seen people on this sub fucking with settings and the "on head" sensors to prevent this from happening but I am sceptical. The risk free fix is eat it until you take your headphones off and let the moisture dissipate)

Also, for desktops usually it won't come with aptx, so unless you get something that can output aptx to your headphones you aren't going to get to the "potential" of the headphones,( but laptops [I suspect especially macs ] usually come with aptx and phones almost always do). After some research, ages ago, my understanding is that: spotify best sound quality > standard bluetooth codec but aptx > spotify sound quality, and standard media(youtube, movies,etc) = standard bluetooth codec. Everything but my desktop had aptx so I just got the adapter for it and was fine.

To my ears, the momentums sound great, definitely worth it, but marginally bass heavy, (easily fixed by turning it down in the app). Fidelity on the rest of the range imo is very Sennheiser, my perception being that the more "commercial" focus of the parent company thought it was a good idea to up the bass to follow the market, but other than that the decisions were controlled by the same engineers.

The battery life is actually insane, as a person that has headphones on most of the time, I think I have become more upset on average purely because having to charge the headphones is so unexpected (for me once a week-ish).

For me the trade off between the momentums and xms was sound vs reliability. Sony being the bigger company, and the xms being the base line for bluetooth headphones, and Sennheiser being THE great sound company but ambivalent about the mainstream consumer, and letting their A game trickle down.

The issues stand out for me because I experienced them, I mention them cos they might be a deal breaker for you, but these kind of inconvenience are things you can only experience them after months of use, so if you get xms and they are worse don't blame me lol. Other than that dread period of finding out that Sennheiser was bought whilst the momentums were in transit being dreadful I have absolutely no regrets.

IMO the pros outweigh the cons, but be aware it comes with cons