ChipFlip
Open-back or closed-back is the first question in headphones, and it decides more than the brand does. The Beyerdynamic DT 1990 Pro runs $325 used, the Sennheiser M3AEBTXL $321.58. Nearly identical money, fundamentally different products.
The DT 1990 Pro is a studio tool
Open-back, wired, analytical. It's built to let you hear exactly what's in a mix, including the parts you'd rather not hear. Beyerdynamic ships it with two sets of pads that meaningfully change the tuning, which is unusual and genuinely useful.
Open-back means everyone near you hears your audio, and you hear the room. That's a dealbreaker in an office or on a train, and irrelevant in a room by yourself.
The M3AEBTXL is a closed travel headphone
Momentum-line Sennheiser, closed-back, Bluetooth. Different job entirely: isolation, portability, wireless convenience. You can wear it on a plane and it makes sense, which is not something you can say about the Beyerdynamic.
The tradeoff is that a closed wireless headphone will never match an open studio reference for soundstage or detail retrieval. It isn't trying to.
Wired versus wireless is the buried cost
The DT 1990 needs an amplifier to perform properly at 250 ohms. Plugging it into a phone or a motherboard's front-panel jack gets you a quiet, thin version of what the headphone can do. Budget for that, because it can add real money to the total.
The M3AEBTXL has its amplification built in and a battery to keep charged. Neither of those is better, they're just different things to manage.
Bottom line
Critical listening at a desk, with an amp already in the picture? The DT 1990 Pro. Anything involving leaving the house, or shared space where sound leakage matters? The M3AEBTXL. At a $3 difference, the form factor is the entire decision.