After working at Windells both winter and summer for several years I can say the winters are awesome. At times I have even preferred them. There are less people so you pretty much have unrestrained access to everything and a closer relationship with all the staff and campers.
In regards to the weather yes it can be less favorable than the summer. It's mid winter in the northwest. Which means it will likely be over cast for part of the time. However since it is the winter and it snows A LOT in the northwest during the winter you are likely to get at least one powder day. The staff at Windells knows all the mountains very well and they can take you to places to ride untouched powder for just about any ability level. If your group is feeling up to it there are even places you can quickly and easily build jumps. It was always one of my favorite things to get kids out into the side country (at timberline you can go about 50 feet off the trail and find amazing gullies and canyons) and hit wind lips, natural hips and trannies, step downs, step ups, etc.
Besides the weather the primary aspect of summer camp that you don't get in the winter is access to the private park and Windell's private features. That being said Timberline has a sick park and since the camp is held mostly during the week there is hardly anyone there. You can hot lap the parks via the highspeed quad.
I highly suggest checking out the winter camps.