I'll choose the male Inkling because, well, I'm a guy. I know that the girl is more popular because she's cute and because many gamers like having a strong female lead, but I think the guys are pretty cool, too.
As for ink color... I'd prefer purple, since it's the closest color to black, which is my favorite color IRL. Of course, I also plan on frequently using green and blue ink, especially with Ink Color Lock enabled for blue. And as far as Octo Valley goes, I'll save Ink Color Lock for the final boss so that I can use yellow ink, since yellow is the complementary color to purple, the ink color used by the Octarians.