The Escoda Ultimo and Aquario are watercolour mop brushes made by Escoda, the brush making company from Spain.
These are short handle wooden brushes with metal ferrules, and they use the same brush size system.
The main difference between the two is the type of hair used. Ultimo uses synthetic hair while Aquario uses real squirrel hair.
Both types of hair are able to hold a good amount of water which makes these mop brushes work really well for painting large areas. It seems easier to retain a sharper point with the real squirrel hair of the Aquario. However, these two mop brushes are not meant for painting details so whether they can hold a sharp point is not that important.
Just like any squirrel brush, the hair is soft and will not return to its original shape after each stroke. There's also no springy feel like sable brush. But all these are to be expected from squirrel hair brushes.
The overall performance for both brushes is excellent. There's no much difference between choosing synthetic or natural squirrel hair in this case.