The one difference is that with the app you get the on screen display. It's nice being able to glance at the screen and see the artist, and do the thumbs up/thumbs down from there. However, the overall awkwardness of having to fire up UConnect Access then having to run the slow-starting car app is enough that I probably won't subscribe once my trial period runs out.Honestly? Just play Pandora through Bluetooth onto the car's system.
I've used the uconnect app and it just seems redundant. You have to launch the uconnect app on your phone anyway so it's not like you don't have to reach for your phone anyway.