Better to excavate in the spring or winter?
tl;dr Insulate the excavation area now and dig later.



Excavating in the winter is difficult only if you don't prepare the site. Lay down insulating blankets and/or straw over the entire area to be excavated prior to freeze. This will keep frost to a minimum and allow excavation at any time, such as when contractor prices are lower. Even if you're late and miss the freeze, insulating later allows ground heat to thaw some of the frost. 

Excavating early means that the frost now exists at footing/slab level when it comes time to build. This presents its own set of complications, such as difficulty burying plumbing and drain tile and having to pour concrete on frozen ground. Better to have freshly-dug soil at that point, get footings and plumbing laid, and backfill.