I voted for both.
My preference, where possible, is to buy the CD. Then I rip it to mp3, store it on my NAS, and the music is available to any computer, DLNA device or my Sonos network.
When that is not possible, or if I'm just too impatient, I will occasionally buy tracks or albums from iTunes. (Amazon mp3s are not available in Canada.) I convert the AAC files to MP3, then file them away with all the others.
Purists will decry the double conversion from AAC to MP3, but frankly, I can't hear the difference with the equipment I've got. Should I one day upgrade to a higher quality listening room and discover that I can hear the artifacts and am bothered by them, I'll just repurchase the offending content.
The nice thing about CDs is that you can buy them used. Even when new, the cost is often only a couple of dollars more than the iTunes album.