Blankets are tricky, because most horses don't need one. They are happier in colder weather, and their hair does a better job of keeping them warm then most blankets.
Consider this: The weather gets REALLY cold, and you bought a lightweight or medium blanket because you want to help your horse out 'just a little' and it makes you feel better. She has a good coat, so she doesn't need a heavy weight, right? Your horse keeps it's body warm by trapping heat between it's body and it's hair--that's why they look extra fuzzy on cold days. Their ability to stay warm depends on how much of their coat can fuzz out, pretty much. When it gets seriously cold and you have a lightweight or medium weight blanket on your horse, the only thing you accomplish is to make your horse cold. Lighter blankets in cold weather do not help; they push the horse's coat down so that the horse cannot insulate himself. And relying on a lightweight blanket in freezing cold weather does them no good!
Lightweight and mediumweight blankets are good for mild weather on a show horse with a light coat or one that is clipped. If you are thinking of getting your horse a coat to minimize her own fluffyness, or to make grooming easier, you should get a heavyweight--amigo heavyweights run cheaper then most, and they are still a top-notch blanket. If you are thinking about getting your horse a blanket because you 'feel bad', there's nothing to worry about as long as she has somewhere to get out of the wind. Horses are happy without one, and safer (nothing to catch on them) too!
Hope this helped. Good luck!