butter vs. margarine
Most of the reading on this matter recommends that you avoid both butter and if you must choose, then go for a margarine made with vegetable oil and containing omega 3.
But not all margarines are created equal. Some vegetable oils are healthier for you than others - or should that be, some vegetable oils do you less harm than others. Also, the process of creating margarine can introduce "trans fatty acids" that can raise the LDL cholesterol (the "baddie") and lower the good HDL cholesterol.
Personally, I prefer the taste of butter and I choose to use butter over margarine in my cooking. OK, butter is all animal fat and that's not good for you in high quantities. However, I really don't know what the other chemicals are in margarine so at this time, I avoid using it.
Keeping butter soft
Butter straight out of the fridge is difficult to spread and you may use more than you need on your morning toast and sandwiches. Leaving the butter out of the fridge runs the risk of having your butter separate (not pretty and not all that tasty either) or worse still, your butter becomes rancid (off).
I have a solution. Bring a 500g block of butter to room temperature in a mixing bowl and, using your cake mixer, beat until the colour becomes lighter. Slowly add 1 cup of good vegetable oil (canola) until it is all incorporated. Scrape into a plastic container and refrigerate. The butter will become solid, but easier to spread straight from the fridge.
Warning, don't use olive in this process because the flavour overpowers the butter taste.
You can use your spreadable butter quite safely in your cooking.