I have done tons of research on this subject. Because I travel a lot for business, auto top-off is essential as my wife often forgets to add water.
There are positives to both systems. One advantage to the Tunze system is that it doesn't have any moving parts because it uses optics versus a float system. The JBJ system costs about 100 less and also has a built in fail safe.
There is another system to consider at:
Personally, I went with the JBJ system which I really liked for the $$