Tough question, depends on her facial features/structure I suppose. Probably should just walk into a high-end salon for professional advice on this one I'd say. But a blend of both, I mean you don't want too straight, or too arched, both would look unappealing.
Never really thought about it. as long as they look natural and groomed and the shape suits the face, I don't get alarmed or turned on or turned off. I do find bushy a little unattractive and I certainly don't like the very thin artificial ones done with an eyeliner or whatever.
In the first pic, doesn't the nose in the right pic look different from the nose in the left even though it's the same woman? And I chose straight cause that's sort of how my eyebrows are but arched looks really good on a lot of girls. I don't think my face can pull off arched eyebrows so I just leave them naturally straight LOL.