I haven't tried them yet, although I am seriously leaning toward the i-Blason one. They also have it in White. From what I have seen of reviews, people are pretty happy with it, especially for the price. Based on reputation alone, I think Mophie has the edge, but it is hard to argue with a price difference that large, when most of the reviews are so positive about it.
I've only had my phone less than a week, so I am not ready to spring for a power case yet. The battery life so far has been good enough that I don't feel rushed to get something. Since I already use a Defender case, the size isn't a factor for me. The white i-Blason one in particular looks really nice. The kickstand isn't a bad feature either. Personally, I thought it looked kind of... out of place I suppose. Kind of ruins the lines and look of it. The black one the kickstand is nowhere near as noticeable as on the white one.