feynmf vs. feynmp: What's the difference?

What is the difference between feynmf and feynmp? By very basic understanding is that the former somehow makes use of raster graphics whereas the latter makes use of MetaPost.

Moreover, I have read that feynmp is actually preferable to feynmf, but why? Is there something that you can do with feynmp that you cannot with feynmf? Are some things easier? Does it do the same things better? I noticed very little difference when I changed \usepackage{feynmf} to \usepackage{feynmp}. Indeed, the biggest difference was that I had to add an extra line of code in order for it to typeset without error!