Well, the Senn Momentum definitely is a smaller headphone with smaller earcups. It is an over-ear, but only just, so if you have big ears, this is going to be an issue. Other than that, it sounds good and is beautifully made.
The Beyer DT770 (assuming you mean the studio-model "Pro", not the older home-model without the suffix) is a much bigger headphone. Very comfortable to wear. It sounds a bit more loudness-ish (more bass and treble), but it is very well done. The bass is tight and goes down low, the treble is open without ever being aggressive. Only the midrange is a tad recessed. So very nice to listen to, but if you like to have the singer's voices so much in your face that you'll get tears in your eyes, a less loudness-ish headphone is a better choice. The Senn tonally is more neutral in this regard.
One option I personally recommend is, in case you decide for the 770, is to swap the artificial leather earpads for the velours earpads of the 770 with 80/250 ohms. I find velours earpads much more comfortable to wear, also you sweat less with them. However, because they do not seal so good, you lose a bit of bass impact. It's a trade-off, but if the 770 with 32 ohms has more than enough bass for your liking and you like comfort, consider this option.
Both headphones are closed, so expect to get warm ears after a while. However, the 770 is much more comfortable in this regard due to its larger size (not so much heat building up) and especially when you swap the earpads for the velours versions.
If you can live with the bigger size of the 770, I would definitely recommend it over the Momentum, not only due to the lower price. If you want a Senn, I would rather get one of the old bunch like a HD600. However, they are open headphones, so you will hear more of your surroundings and also emit much more noise to the outside, which may disturb some.
Hope this helps!