Milo flows so nicely off my tongue, it's much less harsh than Otto, but it's difficult to imagine an adult Milo in my opinion. Plus everyone online says it reminds them of dogs and chocolate milk, even though I've never seen/heard of Milo powder or met any dogs called Milo (not worried about the Tweenies since I think that association is dying out and will continue to do so in the near future).
Otto, on the other hand, seems like a very cool name. On paper, I prefer it! It's different, and I love that it's palindromic too. It's a strong name. But it doesn't feel as nice to say as Milo does, and I don't know if I can imagine calling my son it.
So, a name I think is cool and manly but hard to imagine on a baby, vs. name I think sounds really lovely and is cute for a little boy but perhaps not great on a grown man?