Although both methods work, I would use the first because using the first method:
1) Browsers which don't support media queries (ahem IE8) will at least have fallback styles
2) Your code is slightly smaller and less messy.
Also, for what it's worth (although not really related to your question) if you're interested in a mobile-first approach you could re-write the css as:
body { color: blue; }
@media (min-width:480px) {
body { color: red; }
}