MRP's typically contain additional vitamin and minerals and a typical macronutrient breakdown of something like 40g protein, 20g carbs, and 2-3g fat. If you use more than a couple a day and are taking a multi-vitamin, try to make sure you're not getting excessive levels of fat soluble vitamins or things like iron. Weight gainers generally have far more calories, which are in the form of maltodextrin. While technically a complex carb, it has much the same effect on your body as sugar due to the fact that it falls quite high on the glycemic index. A typical macronutrient breakdown of a *bad* weightgainer is 50g protein, 300g carbs, 1-2g fat. Better weight gainers--the ones that don't have 2000cal a serving--like N-large2 are closer to 50g protein, 90g carbs, 4-5g fat. While being good for a post-workout drink, at any other time this is quite high in "bad" carbs. Most weight gainers have low levels of added vitamins and minerals or none at all. And the best weight gainer of all: real food.