>Not using both
Both and Altair is the ultimate frontline in water.
4 is better right out of the box, since Uno really needs his 5* to shit out the damage and just has a 100% damage cut otherwise, while 4 still has his delay, gravity and buff extend.
If you want to know which one to 5* first, consider this:
Do you want insane damage for a turn on a rather short CD and another Stamina modifier? Do Uno.
Do you want to free up your mainhand slot from the GW dagger and give AIDS to your enemies? Pick Feower
Do both if you have Varuna and want to compete with Titanlords