The difference is mostly related to the context in which each is used.
Both words, population and populace, derive ultimately from a Latin word for people, but the words entered English with distinctive meanings. From the beginning, population referred to the collective inhabitants of a place, whereas populace had the meaning “ordinary people” as opposed to the titled, wealthy, or privileged classes ... Not only did the word populace refer to the less privileged part of the population, it was also used as a pejorative term for “the mob, the rabble, the unthinking masses.” source
In things like scientific articles or any similar context when talking about some subgroup of people, use "population" or "general population" instead of "populace".
In the United States, researchers calculated that up to 15% of the general population are at risk of developing heart disease over the next ten years.
Because of its nuanced meaning, I recommend not using "populace" until you fully understand where it is appropriate. For example:
The jury quickly agreed on a verdict of not guilty, and the acquittal was greeted by the populace with shouts of triumph.
In modern writing it may be more common to refer to "the people" rather than "the populace".
It is doubtful that Mr. Nixon or any other thoughtful person could accept "what the people will support" as a valid basis for foreign policy decisions.