When you buy a CD you don't get the MP3 you get access to a digital version of it on Amazons cloud drive. This means the track can only be accessed whilst you have a connection (possibility only through an android device), its only available at 256 kbps or under bit rate which is the minimum you'd expect and has DRM that limits it so its only available on 10 devices.
If you buy the MP3 you get the actual file DRM free, you can play it on any device offline, the bit rate is what ever amazon sell it as, but they are pretty bad and not telling you before you buy it and its never above 320 (so probably not much of a win there). You'll also probably get better artwork and maybe a video or something else like that.
Buying the CD and ripping your own losses version is always going to be better