[name_u]Love[/name_u] them both. [name_u]August[/name_u] is on my list too ([name_m]Otis[/name_m] might be too but a friend has an [name_m]Otis[/name_m]). [name_m]Otis[/name_m] goes well with siblings names but I don’t think that should be the most important thing.
With [name_u]Dexter[/name_u] and [name_u]Arlo[/name_u], I would choose [name_m]Otis[/name_m]. [name_u]Roy[/name_u] is one of my very favorite names (my grandfather’s name), and I think it sounds great as a first name with brothers [name_u]Dexter[/name_u] and [name_u]Arlo[/name_u], as well. Good luck!
I love both, but in the end I’d go with [name_u]August[/name_u] because of the nicknames [name_u]Gus[/name_u] and [name_m]Augie[/name_m], which I love.
My vote is for [name_u]August[/name_u]! [name_m]Otis[/name_m] is cute, but I feel like [name_u]August[/name_u] is a name that is easier to grow with - it’s one of my favorites!