I definitely prefer [name_m]Dashiell[/name_m]! I mentioned [name_m]Zayden[/name_m] to my parents once and they both said it sounded like “Satan” but trying to be trendy, and I’m afraid I’ll never be able to unhear that now.
I don’t mind either, though I recall the -aiden names being very trendy in ~2015.
Dashiell is adorable! [name_u]Dash[/name_u] is really sweet as a nickname too!
Dashiell has deeper roots tied to literature and also has a sweet bonus nickname—[name_u]Dash[/name_u].
[name_m]Dashiell[/name_m] 
I LOVE it. It’s cool and on-trend, but feels distinguished, too. It’s got literary links, but also the cute Incredibles connection. It’s grand in full, but [name_u]Dash[/name_u] is cool and modern for short. It just feels really versatile.
I like [name_m]Zayden[/name_m] better myself. Or even just [name_m]Zayd[/name_m]. It is very “on trend,” but I really like the sound of it, and I do see it aging well. I have no objections to [name_m]Dashiell[/name_m], but the sound appeals to me less, and actually I hear of it being used more than [name_m]Zayden[/name_m].
Zayden sounds very trendy to me, though I do like the idea of the nickname [name_m]Zayd[/name_m].
I think [name_m]Dashiell[/name_m] is cool and interesting but you might have to explain the spelling/pronunciation sometimes. When I first encountered it, I thought it was pronounced [name_u]Dash[/name_u]-ee-el. [name_m]Just[/name_m] something to be aware of.
Definitely [name_m]Dashiell[/name_m]! It could work well on a child, teen, adult or old man whereas [name_m]Zayden[/name_m] seems hard to imagine in a professional situation. It’s not stuck to one generation and also has the cool nickname [name_u]Dash[/name_u].