The filler doesn't make that much difference - it is the design of the waterways when you push the lever to flush that makes the difference, filling the tank back up at the end is sort of anti-climatic. Note, the right flapper valve does matter - get the wrong one and it may not use the right amount of water, defeating some of the water saving design or performance. Course, some brands use different sized holes and other brands/types of fillers won't work on a particular toilet. My unprofessional opinion.
