I switched to dark brown sugar in the little Hagen cannisters when I was using them. It seemed the bubbles started faster, and had greater frequency. However, when the brown sugar died, it just died--no slow down.
When I started using the 2 liter bottles, I played with the Nyberg method. One of the optional ingredients is molasses, which I believe is part of the coating on dark brown sugar. I've finally settled on mostly white sugar with a couple of tablespoons of brown sugar mixed in.
My final recipe isn't that close to Nyberg any more, but it makes me pretty happy (probably not the best recipe, but works well for me).
For a 2 litre bottle:
1 1/3 C white sugar
1-2 T dark brown sugar
water to within a couple of inches of the top
1/2 a multi-vitiamin (optional, but seems to increase longevity)
1/4 (rounded) tsp yeast dumped on top after the bottle has been mixed.
Let it sit for a few hours before connecting--it seems to work better that way.
I also just dump the liquid from the bottle and leave the sludge in the bottom.
btw--i buy the old, hard, brown sugar that's on sale and it works fine.