My personal opinion? I'd rather eat butter and limit it's quantities than eat margarine. More and more studies are showing that (a) dietary cholesterol has little effect on serum cholesterol and (b) natural fats are better for you than processed fats.
My life includes cutting out as much processed food as possible. A rule of thumb that's been mentioned on here before is to eat "one ingredient" foods - or something close to that. That means your food is as close to what nature intended it as possible.
From the Land O Lakes website:
Butter: INGREDIENTS: Sweet Cream, Salt. CONTAINS: MILK
Margarine: INGREDIENTS: Liquid Soybean Oil, Partially Hydrogenated Soybean Oil, Water, Buttermilk, Contains Less Than 2% Of Salt, Soy Lecithin, Sodium Benzoate (Preservative), Vegetable Mono And Diglycerides, Artificial Flavor, Vitamin A Palmitate, Beta Carotene (Color). CONTAINS: MILK AND SOY
Personally, I know which one I consider the healthier product.