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At least based on what you said, I can see why it would be difficult to choose between these two characters. They're both appealing for very different reasons.

I'd personally go with Ken Kaneki, if for no other reason than because he seems a bit more unique (by Omniverse standards) and has more range. That said, I think Ruby Rose would be a better choice if you see yourself interacting with other characters a lot, especially other "hunter-esque" female characters like Mami.

EDIT: Just to be clear, again, I am not familiar with either source material. I am solely going off of what you presented here.