We recently converted all out toilets to dual flush. We started with basic low flow 1.6 GPF units.
We used two different conversions, Two flush.com and Selectaflush.
We did the Two flush first and found the applications very limited. If your toilet does not fit in a very small range of types, it won't fit. I had to modify the internal arms drastically to make it fit. That being said it works great! You can really fine tune the amount of water you use and get it to a minimum.
For the other two I used Selectaflush, they were substantially cheaper and seemed better designed.
The installation is much easier the design seemed better. By the time I was done I was sold on this model. Until I flushed it. the Selectaflush introduces the water at a much slower rate, giving the small flush very little power to properly flush. It appeared that the large flush may use even more water than without the conversion.
So the end result, if you are a little handy and can make the Two flush fit, the performance is stellar.
I will be removing the other two.