I grew up in Michigan and spent some really cold winters there. Here in CA I tend to wear shorts almost year round, but in the winter there are those unpredictable days when you need a pair of pants. When we go to DLR in the winter I usually end up wearing shorts during the day and bring along a hoodie, then wear a pair of jeans and the hoodie at night.
It really becomes a personal choice, because cold for me may not be cold for you and vice versa.
Bring both and be prepared to change in the evenings, although you might not need to.