Ok, very hard decision my friend,
The momentum are just about the best music headphones, they really bring your music to live with great mids and lows and perfect base for my taste, not booming like dr.beats etc just perfectly present. Confortable fit.
Now, the mm550's are fully packed with technologies, Bluetooth, noise cancelling, srs etc. Of course no wires is great, noise cancelling works good except it cancels about 70% of the base (can go around this by cranking the volume and using srs which brings back some base) , music under blutooth as good as wired, very nice sounding, mids a bit flat but very good anyhow. Also the volume maximum is not what your use too in that price range. at first Ithought of returning them finding I could not pump up the volume as high as my monentum. But after some getting use to them, I very happy and learned that sound is great without that need to listen to high volume all the time. Very confortable fit.
So if you want a pair of headphones mostly for music listening - Momentums are a great choice.
If you want headphones with great technologies, portability and still very good sounding headphones, go with the mm 550's.
I'm lucky enough to own both and enjoy them equally depending of my needs for the day. But again if it's mainly to enjoy your music, the momentums will make you very very very happy.
Good luck, can't go wrong with any of them