I am always a fan of not blanketing if at all possible as I believe if they are able to grow their own protection, and have access to a shed/shelter, they should be fine and it is better for them. It actually inhibits there coats function to cover it and flatten it out. As we have all seen when it gets cold the hair shafts act as a sort of down/solar panel that stands up and draws the sun. That's why they get that "fluffy" appearance As they get warm they flatten the hair down as in after a workout.
If horses are confined, I tend to blanket based on temperature and individual need. Let's say an older horse, a thin skinned horse, a sick horse or a horse that has come from a warmer climate recently may need some additional help. I also look at breeds. Some are tougher then others. Have they lived outside and been allowed to develop their natural protection. Some are clipped for competitive/training/cooling reasons. I believe also, that if they like us, cannot move around naturally and keep their circulation going (as in turnout) they could get cold.
If you must blanket, I am a loyal Rambo/Rhino/Amigo (their least expensive one) fan. They are durable, fit beautifully, never shift, never cause shoulder rubs (I don't know what the secret is), Have minimal straps to deal with (NO leg straps which saves time and I think the horses like more) They even make a QH specific cut line. But I never found it necessary.
Of Course I now own Mercury who has made it VERY clear that he wants to be NAKED! He refuses to wear anything and lets me know by destroying it within minutes and turning it into his personal toy running around gleefully with it like a puppy playing with a chew toy. Yes, I have learned the hard way as I watched him destroy brand new blankets in minutes. Now he only get's blanketed if absolutely necessary.