The debate is which is worse for you.  Butter proponents contend that margarine is an unnatural, man made chemical concoction that has nothing to do with real food. "You might as well be eating plastic!"
    Though there might be some truth to that, butter is milk fat. It's very high in the worst kind of cholesterol.  It will harden and clog you arteries leading to a heart attack or stroke. 
      For many decades I chose to go with the more heart healthy margarine.  I would buy butter for some dishes that just don't taste right without it like Cream of Mushroom Soup.  But 99% of the time I used margarine.
      My sense of time sucks but maybe a year ago I decided to cut way back on my margarine use.  When I started to do this I was surprised to find that I could cut it out completely.  I thought I would miss it badly but I didn't/don't.  For frying eggs or for recipes that call for butter or margarine I use extra virgin olive oil.  For many of the things I used to put margarine on I now use soy sauce.  I only buy butter or margarine now if I'm having company over. 
   So anyway which do you use and why?

Seems that so little is really definitively known about nutrition even today, so I just choose the best tasting variant 

I do think butter and fats have been excessively demonised, but I also am not sure I'll go all the way down the bro science route until more evidence is provided.

Problem with all these butter-alternatives isn't the nutritional value, but the ingredients. You turn over the packet and there are 10 or so things in there, half of which you don't understand (often manufactured in a lab), that can do your system real harm. Same goes for fake meat and milk.

Whereas with whole foods you know what you're eating, only 1 ingredient on the label.

With synthetic food becoming common while fresh, natural food is becoming rarer, I say we should support the farmers and producers of natural food and buy their products, lest they're replaced completely.