Use px for an exact pixel font size, or simply a percentage based on the main font size, so all your font-sizes are linked to the main font-size, useful when changing the base font size, you don't have to change all other font-sizes!
Example:
html{
font-size: 18px;
}
h1{
font-size: 200%;
}
h2{
font-size: 150%;
}