I just purchased exactly the same song from iTunes and Google Play Music to compare them. If you are interested, here are the differences.
iTunes uses AAC, while Google Play Music uses mp3. Google Play uses a slightly higher bitrate, but that probably compensates for AAC being a better algorithm. I can’t hear any audible difference. Read on to see the actual specifications of each music format.
Google Play Music
MPEG Layer 3 (mp3) Stereo 44.1Khz
AAC Stereo (L R) 44.1Khz
275.5 kbits/sec (I assume it’s variable bitrate)