Zanis is much much squishier, is severely lacking in the cc department, lacks mobility and his gap closer with squishies pales compared to zephys, has a really slow jungle clear speed and the only redemption is that his damage output is quite a bit higher but only in the second half in the game, considering zephys already shreds squishies this is only really usefull vs extremely tanky line ups. And vs such line ups the true damage also starts to shine.

In the midgame an equally skilled zephys should be far ahead of zanis because his early game jungle clear and objective taking and ganks are so much better, if your opponents are feeding zanis snowballs really fast though