I was extremely lucky to get to test the InsideOut. I love it and if you want a read then my full review on it is here.
In terms of the difference between it and the Duo (which I haven’t tried), I think the Womanizer clitoral part is exactly the same, same power, same number of levels, no difference. The Duo however, has a slightly longer and significantly girthier arm. They both come with a 5 year warranty and I love my InsideOut and am not going to spend money replacing it as it does it’s job brilliantly but if buying from scratch, if the prices were equal, I would go for the Duo just for that extra girth though don’t need the arm any longer than it is on the InsideOut.
I think, like all rabbits, they are anatomy dependent and the two are similar in who they’ll fit well. For me they’re perfect for others the fit isn’t quite right.