It always seemed to me that the summer deck was more about planting and building, while the winter deck was more about harvesting and fulfilling wine. So it seemed there was a big difference to me, but maybe that was just my perception.
That pretty much sums it up.
Summer visitors concentrate more on the Summer (and Spring w/ Tuscany) actions. Winter visitors concentrate more on Winter (and Fall) actions. There are a few that allow you to do something in the opposite half of the year, but they are the exceptions.