I think both are rising in popularity. I do like
Ariel on a boy, but grew up in the 80s and 90s so the mermaid pull is too strong for me to seriously consider it. I do know a couple of Ariels that are guys, but none of them are from the U.S. or had any Disney association with the name.
I think
Arielle is pretty but bring the new range of pronunciations with it. I know a newborn
Ariella and they pronounce is are-ee-ell-uh.
Both are beautiful names, but the
Arielle spelling does disassociate with the mermaid which is probably a plus.
I would go with:
Ariel Zane for a boy to be trendy, or
Ariel Heath to be cool.
For a girl -
Arielle Willow is trendy but smooth and
Arielle Cate would be classy, while
Arielle Rose would be very Disney and
Arielle Wren sweet. I think I like something more elaborate myself, like
Arielle Valentine.