Uninstalling the sideloaded app will require a PC with ADB. Hook up your Glass to the PC and type in the following command before you install the official app: adb uninstall com.google.android.music.
If you want to see what Google Play Music looks like on Google Glass, check out the slideshow below. If you want to hear what Google Play Music sounds like on Google Glass, you'll have to buy a pair for yourself.
Google Play Music officially comes to Google Glass