Well I only have the Siri so I can't speak for the comparison. But I will say that the fact that the Siri is not even splash proof really makes cleaning it a bitch. So being waterproof makes the Alia pretty damn convenient. And the Siri does become difficult to maintain a handhold on if your hands get even a little lubed up so the hand hole of the Alia also intrigues me greatly.

I'd like to make a bump on this: anyone own both and can compare? The waterproof of Alia interests me, but if that's at a sacrifice of power...

I've found that I actually think my Alia has deeper, rumblier, and stronger vibrations than my Siri does. My Siri's vibrations seem buzzier. I actually really liked the Alia - the button configuration was better, it was easier to hold, and it was just a great toy. You definitely aren't losing any power - not in my opinion.