It's a personal choice, really.
You will find some people on here who would say "no, don't blanket" and others who will say you should.
IMO, there is no right or wrong answer.
PERSONALLY, I do blanket when they are outside( we don't have a run in), even my big fat hairy draft. And I live in southern PA where we rarely go below zero. I also stall them during ice storms or blizzards.
Since you have decided to blanket, and he DOES have a run in and you only want to buy one blanket.....I would probably go with a lightweight or medium weight....i definately wouldn't invest in a heavyweight as you will only be able to use it during extreme cold...so it's use would be limited.
Lightweights usually are around 180 grams of fill or thereabouts.
Medium weights usually are around 220 grams of fill or thereabouts.
Since your horse grows a nice thick coat, has shelter....I would think that the only thing he really needs from a blanket is waterproofing...since he likely won't stay in the run in constantly and will likely wander out of it even if it is snowing or raining.... My personal opinion is that considering what we know of your circumstances, a lightweight would be best... certainly, I would NOT buy the blankets with 300 or 400 grams of fill.
Rambo blankets seem to be the best, but they are very very pricey. Same with Kensington.
Just remember: water resistant or water repellent IS NOT the same as water proof.
More important than brand name is that you want something 100% WATERPROOF.