It's one of the hottest food debates of modern times - which is better, butter or margarine? When it comes to taste, I, like many people, prefer butter. Taste aside, what about the nutritional value?
Ideally it would be best to have neither, as they both provide extra kilojoules / Calories (energy) - unneeded by most people given that 60% of the Australian population are overweight or obese. Keep in mind however that the kilojoules your spread provides is fairly small compared to that in the bread you're spreading it on, with the average serve (teaspoon) of butter or margarine providing 120 kj, whilst a slice of bread provides around 760 kj. In other words, you'd be consuming fewer kilojoules by having 1 slice of buttered bread than you would by eating two slices of plain bread.
If you do need to use a spread and you are watching your energy intake, be careful only to use small, conservative serving sizes. Read more....
Risk of Heart Disease
When it comes to which is the lesser of the two evils in regards to the risk of heart disease, there is no clear cut answer as there are various different types of margarine's - some better than others. Butter is generally worse, as it is a rich source of saturated fat which raises blood levels of LDL 'bad' cholesterol, which is associated with a higher risk of heart disease.
Depending on where about's you live however, many margarine's can be just as bad if not worse, as they can contain high levels of 'trans fats' - a type of fat considered even worse than saturated fats when it comes to heart disease. Fortunately for those of us living in Australia, the trans fat levels in Australian margarine's are very low by comparison with those in some other countries such as the United States.  Aussie consumers should still look out for margarine's with levels of trans fats as low as possible. People with high cholesterol should choose margarine's with added plant sterols or stanols which can help reduce cholesterol absorption.
Which type is best?
Even though margarine may be better for you than butter (at least those on the Australian supermarket shelves are) not all margarine's are created equal so to speak. As I already mentioned, those with added sterols or stanols are useful for lowering high cholesterol levels, though most margarine's do not contain these substances, and those that do are considerably more expensive. Consequently, unless you do suffer from hyperlipidemia, these more expensive brands may be unnecessary.
People watching their salt intake (possibly due to high blood pressure) should also be aware that salt is a major ingredient in both products, and should consequently choose salt reduced versions and use them conservatively.
Most margarine's contain either canola or sunflower oil as the dominant ingredient. Those made primary from canola oil would be preferable over their sunflower equivalents, as canola is a source of Omega 3 fats, whereas sunflower oil is a major contributor of the Omega 6 fats.
It is thought that during early human evolutionary times, the ratio of Omega 3 to Omega 6 fats was significantly different to what it is now, as our intake of Omega 6 fats has risen enormously throughout recent times. The current hypothesis is that by improving this ratio by increasing intake of Omega 3 fats and decreasing that of Omega 6 fats may have modest health benefits - including a favourable effect on inflammatory conditions. Read More....
Keep in mind however that the type of Omega 3 found in plant sources such as canola oil, linseed (flax) oil and walnuts (called alpha linolenic acid or 'ALA') is not as beneficial as those found in fish oils and seafood (DHA and EPA). Read More.....
But margarine is almost plastic
Several years ago a nonsensical email hoax began circulating the globe, making misleading claims about the differences between butter and margarine. One of these claims was that "margarine is but one molecule from being plastic". This kind of logic would seem silly to those of us with a background in organic chemistry - something which unfortunately, most people reading the hoax mail would not have, which is why being 'natural' is often a common logical fallacy used to appeal to people who would assume that something more 'natural' is healthier than something more 'synthetic'. This particular logic would be like saying that we should not breathe in Oxygen (O2) because it is but one atom away from being Carbon Monoxide (CO) - a deadly gas. Even slight changes in a molecules composition can result in extremely different substances. Suffice it to say - this claim is silly and means nothing. Read More....
Does margarine cause blindness?
Two population studies in the Unites States observed a higher risk of macular degeneration among people who consumed greater quantities of vegetable fats [2,3]- margarine being on obvious contributor. It has been suggested that higher levels of trans fats may be responsible for this effect, though another population study in the U.S. found no link between vegetable fat and macular degeneration - it actually showed a higher risk among those who ate butter. 
As I mentioned previously, the levels of trans fats in American cooking oils and margarine is much higher than that found in Australian products. In fact, two Australian studies found no relationship between the intake of vegetable oils and macular degeneration [5,6] - the latter actually finding a greater risk among people with higher intakes of animal fats. Read More....
The bottom line is that Australian margarine's certainly do not increase the risk of macular degeneration or any other kind of blindness. Australian current affairs program 'Today Tonight' certainly didn't let the truth get in the way of a good story back in July 2004, when they reported a segment on how vegetable oils supposedly increases the risk of macular degeneration.
Today Tonight only mentioned the studies in the U.S. which showed the link, but did not mention those which found either no association or a protective effect. Moreover, they did not mention the significant difference between trans fat levels in Australian foods compared to American foods.
They interviewed Dr Paul Beaumont from the Macular Degeneration Foundation as their only source of supporting information. When I called the Macular Degeneration Foundation however they told me that despite the media attention, they had no official position on vegetable oils, that the evidence was mixed and that more research was needed to make any kind of link between dietary fat and macular degeneration. The Macular Degeneration Foundations web site makes the following statement:
American studies have shown that patients eating a lot of vegetable oil, margarine, processed and baked foods containing more oil developed MD and lost vision more rapidly. There is disagreement as to whether this relates to the Australian public. At present, cold pressed olive oil appears to be the safest. Further research is needed to determine what effect oils have on eye health. Contact your attending doctor for further advice.
The Today Tonight story was deliberately exaggerated to create the fear that 'vegetable oil is everywhere' and is 'sending us all blind'. Although these were not the precise words Today Tonight used, this is essentially the message that many viewers received, and is one of the most common concerns people convey to me.
As a general rule of thumb, any kind of health related information you see on shows such as Today Tonight or A Current Affair should be taken with a grain of salt, to say the least. Any kind of health related information (or any information at all for that matter) you recieve through email fowards should be completely disregarded (or see snopes.com to check its validity)
 Mansour MP, Sinclair AJ. The trans fatty acid and positional (sn-2) fatty acid composition of some Australian margarines, dairy blends and animal fats. Asia Pac J Clin Nutr 1993; 2: 155-163. <full text>
Seddon JM, Rosner B, Sperduto RD, et al. Dietary fat and risk for advanced age-related macular degeneration. Arch Ophthalmol 2001; 119: 1191-1199. <PubMed>
 Seddon JM, Cote J, Rosner B. Progression of age-related macular degeneration: association with dietary fat, transunsaturated fat, nuts, and fish intake. Arch Ophthalmol 2003; 121: 1728-1737. <PubMed>
 Mares-Perlman JA, Brady WE, Klein R, et al. Dietary fat and age-related maculopathy. Arch Ophthalmol 1995; 113: 743-748. <PubMed>
 Chua B, Flood V, Rochtchina E, et al. Dietary fatty acids and the 5-year incidence of age related maculopathy. Arch Ophthalmol 2006. In press.
Smith W, Mitchell P, Leeder SR. Dietary fat and fish intake and age-related maculopathy. Arch Ophthalmol 2000; 118: 401-404. <PubMed>