Scyllion1350 is neutral with low tight bass. t51i has boomier bass and a little brighter. Physically the t51i doesn't have the headband that splits, has softer more forgiving ear pads and an iOS compatible remote.
You can swap out the 1350 ear pads for the t51s. I haven't tried it, but a lot of owners say it's a great mod for comfort, also nets a bit more bass.
I was using Momentums for travel, but after trying the 1350s. I think the Momentums will live at work now. Just need a good travel amp...