|my question is, what is best for filtering in the water tank?|
Quite a clear answer (no matter which manufacturer): none!
The parts are all very expensive, occasionally clog, occasionally leak (granulate leaks). If you have so much hard water that it's stored in a corner as bar stock, you should either use bottled water or an external filter system.
The simple jugs are cheaper than the built-in ones, and if you want to spend a little more at once you can have the big brother installed under the sink. This is also enough for tea and pasta water.
Has less annoyance and costs, and can also detect defective filters before contaminated water is further processed by them.
Quick Mill 05000 "Unique Edition", WMF Ecco