Usually you only go on-hit if your champ has on-hit in his kit, like kogmaw W, vayne W, kaisa passive or varus W
Crit is basically for everone that lacks on-hit.
That said, the on-hit champs also can choose to go crit
The main differnce is that on-hit has better 1 and two item spikes with
and
and is strong in drawn out fights due to the consitent dps and sustain and the ramp-up from rageblade.
Crit is more about bursty damage spikes where you go for single hits, and if one crits you can deside follow up. As an reliable dps tool, it needs longer to get online, usually around 3 items, but it scales harder on most champs.
Then there is also the thing with ER, which means there is a sub category in critbuilts for champs that like the mana/cdr and favor AD over AS like ashe.
For many adc, mutiple builts can work with differnt purpose. Varus for example can go on-hit for high hybrid dps, a strong 1 and two item spike and good tank shreding, but his kills won't hit as hard and his damage vs squishys is a bit lower.
He can go Lethality and focus on his Q poke, but lacks dps vs tanky opponents
Or he can go crit with ER, have still good Q poke but also high dps, but needs longer till he really is effective compared to the other two.
Vanye can go crit to chunk squishys with her Q and basically assasinate them with the stealth of her ult, or go on-hit to shred through any target nop matter how tanky cause of her %hp truedamage.
As for Kallista, she usually goes on-hit/lifesteal.
+
is her bread and butter, and then she can expand with any of those
.
Her AAs do reduced damage, so her crit scaleing isn't that great. However, her E is basically a delayed on-hit which works with hurricane and rageblade, and it't where most her damage comes from.
Usually she goes high sustain cause she has no real burst, she needs extendet fights, and becuase of her passive. She's a hard target to focus and pin down, so if gets alot sustain it really becomes hard to kill her.