Hey
I would say that the print quality of the UM2 is better than the UM1. Also, from my experience, the UM2 is more reliable and easier to use. I seem to have had fewer failed prints with the UM2 (though that could just be me / I had a dodgy UM1).
Either way, if you get a UM1 or a UM2, I don't think you'll be disappointed!