Both are pretty ugly.
Both aren't all that cute, but Neorantu sounds cuter, so Neorantu.
... Neither? They're fish! Can there even be cute fish?! But if I had to pic, I'd say the first form, 'cause I like the color, and colors can be cute. Yes.
Actually, Neoranto (or Neorant, whatever) IS the correct way of spelling it. ^^;Yea! some people misspell it, Neoranto....