From a Cochrane Review meta-analysis of randomised clinical trials, replacing saturated fats with polyunsaturated fats reduced the risk of cardiovascular disease events such as myocardial infarction, stroke, angina, heart failure, peripheral vascular events and atrial fibrillation. Reducing saturated fat intake lowered blood cholesterol and cardiovascular events.
Butter contains 70% saturated fat, 26% monounsaturated fat and 2% polyunsaturated fat. Monounsaturated margarine (e.g. canola oil, olive oil spreads) contains 23% saturated fat, 50% monounsaturated fat and 26% polyunsaturated fat. Polyunsaturated margarine (e.g. sunflower oil spreads) contains 22% saturated fat, 33% monounsaturated fat and 46% polyunsaturated fat.
In the 1950s margarine was formed using the process of partial hydrogenation which created trans fats. Trans fats are known to increase blood cholesterol. However, this process has been replaced by inter-esterification in the 1990s which does not create trans fats. Hence, the better option from current evidence is margarine. Remember to use healthy oils in small amounts :P
Source:
Cochrane Database Syst Rev. 2015 Jun 10;(6):CD011737. doi: 10.1002/14651858.CD011737. Reduction in saturated fat intake for cardiovascular disease. Hooper L1, Martin N, Abdelhamid A, Davey Smith G.
Prog Cardiovasc Dis. 2016 Mar-Apr;58(5):464-72. doi: 10.1016/j.pcad.2015.11.006. Epub 2015 Nov 14. The Evidence for Saturated Fat and for Sugar Related to Coronary Heart Disease.
DiNicolantonio JJ1, Lucan SC2, O'Keefe JH3.
Hruby, A. and Hu, F. B. (2016), Saturated fat and heart disease: The latest evidence. Lipid Technology, 28: 7–12. doi:10.1002/lite.201600001