I use and love my Gossamer Gear Mariposa. For me, it is the perfect size for my gear and a week's worth of food and is comfortable to carry. This is my second Mariposa. I first bought one in 2010, which was still very serviceable, but I liked some of the new feature changes so I got a new model earlier this year. On a section hike last month in Massachusetts and Vermont, I met a northbounder who had been carrying the same new version of the Mariposa since Georgia and he was very complimentary of the pack.
I have no experience with the ULA to offer a direct comparison.