A burning question when you are not aware of why to chose one over the other. I speak purely from a software applications usability perspective. Graphics design and impact of serif and sans serif fonts is a completely different discussion.
If you look at a serif font in a large point size like 24, you get to see pointy edges everywhere which make it Serif. Now the grandness of this font morphs into ugliness on a monitor at point size 11 and below.
A typical monitor or LCD's DPI is limited to 75dpi or 90dpi at the most, its so measily low that you would'nt bother to use a magnifying glass to check a fonts edges, they dont exist, they dissappear at that font size, in other words sharp edges and curves are simply rendered as a blur making these font ugly. But Sans fonts shine through in small sizes, for the simple reason they dont have much pointy edges.
Simple thumbrule: if you use a serif font use it for size above 12pts, which would usually be headers and large elements. For paragraphs stick to sans, works best in small font sizes.