The difference I believe came with the ability for the app to automatically be run in the background and for control over which one was used for Tap and Pay, which given today's announcement from Google makes sense.
Maybe this was fixed in Android 4.4, but I would swear the experience changed for me sometime right at or after 4.4.
Notably, I didn't have to make sure the app was open and wallet unlocked first, whereas with Android 5.0 it "just works".