An article at MayoClinic by an R.D. says:
Margarine usually tops butter when it comes to heart health. [...] So skip the stick and opt for soft or liquid margarine instead.
The American Heart Association says:
Recent studies on the potential cholesterol-raising effects of TFA have raised public concern about the use of margarine and whether other options, including butter, might be a better choice. Some stick margarines contribute more TFA than unhydrogenated oils or other fats.
Because butter is rich in both saturated fat and cholesterol, it's potentially a highly atherogenic food (a food that causes the arteries to be blocked). Most margarine is made from vegetable fat and provides no dietary cholesterol. The more liquid the margarine, i.e., tub or liquid forms, the less hydrogenated it is and the less TFA it contains.
The best choice for your health is a liquid margarine.
Other research (summarized in this editorial from the British Medical Journal) says:
Calder says the findings argue against the "saturated fat bad, omega 6 PUFA good" dogma and suggest that the American Heart Association guidelines on omega-6 PUFAs may be misguided. They also "underscore the need to properly align dietary advice and recommendations with the scientific evidence base."
That quote refers to this study:
[C]linical benefits of the most abundant polyunsaturated fatty acid, omega 6 linoleic acid, have not been established. In this cohort, substituting dietary linoleic acid in place of saturated fats increased the rates of death from all causes, coronary heart disease, and cardiovascular disease.
I think these quotes establish two mutually exclusive claims:
- Margarine (especially soft or liquid margarine that is low in TFA) is better for heart health than butter.
- Death rate due to all causes, coronary heart disease, and cardiovascular disease increases when substituting liquid margarines in place of butter.
Which is true?