I've been using the xtremeguard screen protectors for both the front and back glass since my 5th day when I shoved my N4 into my jeans pocket and the factory screen protectors (with labels and all) came off. I can't speak for the Zagg never having used them, but the xtremeguard does add a little bit more grip during use (which can take getting used to especially when compared to naked screen).
Coming up on 2 months with the xtremeguard, I have no complaints and recommend them to family and friends. I even picked up an xtremeguard screen protector for our N7.