I like them both. I know some people don't like her V3, but it's weirdly nostalgic for me, because her V3 artist (Akio Watanabe) also did the designs for The SoulTaker anime (which has kind of trippy art and makes Rion's design make "sense" to me). If I had to change one thing about her design, maybe I would amke her hair color not so dark purple (too much contrast with her clothes) or her green not so neon.
I think her V4 is really cute and soft colored, I love the art style. The only thing I don't like are her mittens (makes me think of an astronaut). Maybe if her V3 design had the same colors as her V4, it would be really cool?