Is butter or margarine healthier?

Another arguement that i’m having with a friend. (if you read the one about the bandana… lol) Is butter better than margarine? healthy-wise? which is less fattening? And also, if someone was trying to lose weight, is peanut butter a good way to go? I always thought that peanut butter was fattening, but a friend of mine says otherwise. Saying that peanut butter is ‘healthy’, even so, wouldn’t that make you gain weight as well?