Afaik any browser will load both stylesheets, no matter if the media query matches or not. so from a pure performance view the latter alternative should be just a tiny bit better. But from a maintainability point of view i would tend to use the first version anyway :)
I think that the first way is better. While browsers will load both, some newer browsers (latest WebKit) will delay the loading of a stylesheet that doesn’t currently apply until after the page has been shown and everything else has been loaded.