Another point to consider is the type of area you will be camping in. The Shangri-La 3 has a very large footprint that may limit your camping spots, such as in thick underbrush or heavily forested areas. As Paul said, it will also depend on how many people will be sharing it, but I can share with you how I decided on the 3-person version as my solo winter shelter:
1. In the winter, I spend a lot more time inside of my tarp (potentially from sunset ~5 pm until sunrise) and wanted the extra floor space and headroom of the 3 person version. The 2 person version is only 57" wide and the pole in the center cuts the width in half, making it too cramped for me. The additional headroom for the 3 person version also gives more flexibility in moving around, sitting up to read, etc.
2. If you were to suspend the top of the tarp from a tree, the Shangri-La 3 is simpler in that it only has one attachment point, compared to the 2 person version that has two attachment points.
3. There is enough room that I can invite others in to play cards, eat dinner, etc to pass the time between dinner and bedtime. I find that staying awake in camp until a reasonable bedtime is the hardest part of winter camping. The combination of the temperature dropping, your activity level dropping, and darkness make it difficult to stay awake during this time. Unless you can sleep for 14 hours straight, you'll need to keep yourself awake/entertained during this time.