I have both. I have enjoy my Wii Balance Board - and the first time I got to fly on a back of a Banche and shoot at a Helicopter ala Camer's Avatar it was quite amazing. And Wii Fit Plus, it definitely a needed improvement over the original Wii Fit. It's is an interesting add.
But I have to go with Kinect. It effects improvements on so many things - even Skyrim is improved. I see it as a revolutionary device - the augmented reality, voice and movement recognition. It is just a different league all together.
And for a workout enhancer Kinect beats Wii Fit. The board can teach you some interesting aspects about your balance and where you hold your weight. But it requires you to adapt to it - not jumping, or hard steps. While Kinect will use what you do naturally.
I'm Team Kinect for the Revolution!