Which name do you prefer?
Hypothetically speaking, [name_f]Lucy[/name_f] or [name_f]Stella[/name_f] would most likely be a sibling to [name_m]Jasper[/name_m] and [name_f]Ava[/name_f]. 
I prefer [name_f]Stella[/name_f] because it’s not as common, and it fits better with [name_m]Jasper[/name_m]. However [name_f]Lucy[/name_f] provides more of a contrast in the sounds and also doesn’t share an -a ending with [name_f]Ava[/name_f].They’re both very pretty but I do like [name_f]Stella[/name_f] more.
I like both but prefer the sound of [name_f]Lucy[/name_f].
I prefer [name_f]Lucy[/name_f] with [name_f]Ava[/name_f] for the reason @morning_glory explained. Aside from that I prefer [name_f]Stella[/name_f], but I’m biased since that’s my name 
With [name_m]Jasper[/name_m] and [name_f]Ava[/name_f], I prefer [name_f]Lucy[/name_f], as it means that each child’s name has a distinct ending sound. [name_f]Lucy[/name_f] also feels more classically elegant to me. [name_f]Stella[/name_f] is nice, but feels very trendy.
I think [name_f]Stella[/name_f] sounds great 
I [name_u]LOVE[/name_u] the name [name_f]Stella[/name_f] but with [name_m]Jasper[/name_m] and [name_f]Ava[/name_f] I think [name_f]Lucy[/name_f] fits best
This is hard because these two names have the exact same feel to me. I suggested [name_f]Stella[/name_f] to my friend and she picked it for her dog’s name. While I think they are both great with [name_f]Ava[/name_f] and [name_m]Jasper[/name_m], I think I would choose [name_f]Lucy[/name_f].