The only drawback to MiMi is its dang buttons. They're small, metal, and firm so they can be a nightmare to press, especially with slick fingers. The only reason I prefer my Siri to MiMi is because of the button issue, but then, Siri loses points for not being waterproof. I reckon they're both pretty even when it comes to power... MiMi might be a tad more rumbly, but Siri doesn't make my hand as numb.
I also have a Leaf Touch, and the vibrations it produces are disappointingly weak and buzzy. I don't know about the other Leaf vibes, though.
I've heard fantastic things about the We-Vibe Touch, but I haven't had the opportunity to try one yet. It's next on my list.