If your main concern is weight, more than durability, layers matter more. They're flexible and don't actually Add cumulative weight to your load.
For waterproofing, leather is definitely da bomb, but if you can get your hands on a London Fog (lucky thrift store finds, anyone?), those are still some of the best coats around for a reason. They won't keep you particularly warm in their own, which brings us back to layers anyway. And they're durable, too.
As others have said, it depends on your main concern. Real leather, when you find it, is genuinely a great choice, and no heavier than the average work coat. Plus they're usually stylish enough to "dress you up" for unexpected special occasions. But they will NOT keep you warm, in and of themselves.
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