Well, I agree the convenience of MP3s are too good to pass up, but I get nostalgic and feel the need to go out and buy a CD.
CDs = Nostalgia? Man that makes me feel old... Whatever happened to cassettes? (I wasn't around for records or 8-tracks)
In all seriousness though, definitely MP3s. CDs require you to have a CD player nearby. If your car has one, great, but if not, you need to carry around a large (compared to MP3 players) round object. Digital is the way to go, even if you want to buy the songs.