A valve based dual flush will eventually leak, and initially, you probably won't even know its happening. Such a system could lose about 50L per day and most people won't even notice. A syphon flush cannot do this.
Low flush toilets often waste more water than they save due to poor engineering and confusing buttons.
www.bbc.co.uk
I should add that we have a dual flush syphon flush (turbo 88), which works well.