The Kindle DXG is definitely heavier than the Kindle 3 because it is quite a bit larger. The nook is heavier than the Kindle 3 too. But we are really talking about the DXG being about the weight of a small hardcover book, rather than a bag full of groceries (i.e. "heavier" is relative and we are talking about some really light objects here).
One advantage of the Kindle DXG is that it is large enough that it is unlikely you would leave it laying around and forget to take it when you leave an area.
I have the DXG in one of those M-Edge flip top covers and like it just fine. However, I have a Kindle K3 on the way because I do want something smaller for travel. My concern about the DXG is accidentally breaking it because of its size. That may be unfounded, but as much as it cost, I treat it REALLY carefully.
Tony