I agree with everything CP said... I'd just like to add that you should consider both how your horse will treat it's blanket, and how other horses
will treat your horse's blanket!
In my opinion, the higher the denier, the better. 1200 is a good choice.
Also, you have to make sure your horse won't get too hot, or will stay warm enough under it's blanket. They do breath and allow air to get in, but moisture can get trapped under there. When in doubt, go lighter. If a horse sweats under their blanket, it can be very uncomfortable and all sorts of skin issues can pop up. They do a pretty good job of regulating their temperature if they are too cold, so it's better to be on the chilly side. I'd recommend a plain sheet for rain, a blanket with 50-100g fill for mild/cold weather, and a 200-250g blanket for really cold weather. They make blankets with 300g+ fill, but unless you live somewhere that gets consistent snow storms and below zero temperatures, I wouldn't bother with one that heavy.
Breed and coat quality make a difference, too. Stockier breeds usually grow a nice, thick coat, so they require less blanket. Finer boned breeds usually grow thinner coats, so heavier blankets are better. It really depends on the horse, though. I've seen that theory blown out of the water by a few horses!
For example... My horse (a 14 year old thoroughbred with an average coat) wears a 100g blanket anytime the weather is between 30-40*F, and a 250g blanket when the weather is under 30*F. I also take into consideration the type of weather we are having. For instance, if it's warm enough for his lightweight blanket, but it's windy and snowy, I'll put his heavyweight one on. If it's cold enough for his heavyweight blanket, but the sun is really warm, he gets his lightweight blanket... does that make sense?
Smartpak has done a really nice job providing information about blanketing, here are a few articles from their website... Smartpak Blanket Q&A Smartpak Blanketing 101
I don't know what your budget is, but Smartpak is usually a bit pricey. I buy blankets from statelinetack.com and I haven't been disappointed. I currently have two Saxon brand blankets, they are just as nice as any other blanket where I board, but they are half the cost. I am not a fan of Tough-1, though. I don't like the material.
That's all I got! Good luck and happy shopping!