I like Mimi the best, but perhaps because I cannot find another clitoral stimulator that even comes close to in terms of what it does for me personally. I love my other two Je Joue vibes though as well as my Ami. I don't have a Sasi because it doesn't seem like something I would like.
The Uma is *slightly* stronger. But it's barely so.
I'd decide between the two depending on if you want internal or external stimulation. I like the MiMi a lot better for clitoral - the head of the Uma just doesn't work as well for me in that regard - but some people prefer less directed vibrations and like the "handle" on it.
And of course, between the Uma and G-Ki, it's really what's going to "fit" you.