My experience with mids in the winter is limited, but the general wisdom is that silnylon sheds snow quite a bit more readily than cuben. At moderate angles snow may slide off silnylon but not cuben. It'll still collect around the edges and reduce space a bit, but a generously sized silnylon mid should fair well in moderate snowfalls. In heavy snowfalls it's time to get out and shovel.
With that said, cuben doesn't stretch very much, so I've found that I don't lose that much space inside under light snow falls (1-2") but that's with other cuben shelters (HMG Echo, MLD DuoMid), not the UltaMid. There may be something specific to the UltaMid design that is causing this (i.e. guyline stretch). Silnylon will sag quite a bit more for the same snowload. Cuben is probably better for light snows, while silnylon is probably better for heavy snows because it's shedding ability and dynamic stretch.
The weight savings of cuben are nice, but I've recently switched over to silnylon for my mid (MLD cuben DuoMid to Locus Gear Khufu) because I feel more comfortable using silnylon in really cold conditions with potentially a lot of snowfall.
Locus Gear makes a number of really nice mids (Khufu, Menkaura, Khafra) that they'll make in either cuben or silnylon (not all options are listed, so ask). MLD makes great mids in both fabrics too.