My husband and I were discussing this question and I was curious to know if there has been any exploration on this issue. I would think despite it being high desert, the area east of the Sierras would likely have had a decent amount of wildlife such as mule deer going to lower elevations to winter in better weather, plus the Truckee River could have been a decent water source. What compelled the party to stay at the pass as opposed to heading down a bit from where they came?
Whether the Donner Party went west or east, they were stuck in the middle near the summit and would have had to walk out. They would have left everything they owned and would have faced the elements with nothing but their clothing - which was in many ways inadequate. The Truckee Meadows were completely without anything but Northern Paiute and Washoe settlements, and the Donner Party would have feared them - ironically, since they would have probably cared for them if the Americans were not hostile.
From where they were, their best chance would have been to walk west, but either way, there was a good chance they would have been lost in the incessant snow storms and the deep snow. It was the worst of winters.