Overall, Android 5.1 is going to give you the most performance improvement over previous Android OS versions along with much improved battery life. If you can’t afford a new phone but you want to upgrade your phone for free, CM12.1 ROM may be the best way to do that so definitely give these builds a try and do let me know what you think!
(These unofficial builds are compiled from official Cyanogen sources, no bug found so far.)
You will find easy drop-down WiFi and bluetooth shortcuts, which is a new feature of Android 5.1.
With Android 5.1 CM12.1 ROM, you can specify exactly which apps can notify you and also set the priority for these notifications.