Fair enough and cheers for answering a hypothetical question. Physical qualities aside I'd be inclined to go with a right footed Callum Styles myself as I feel he adds more to our team performances than Darnell as a whole. I haven't done any statistical analysis to back this up by the way, it's purely an unconsidered subjective opinion
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In not doing any statistical analysis, we are both on the same page.

I think its to do with how I see the best teams setting up. I like full backs bombing forward in interchange with the wingers. Furlong and Fellows seem to have a good understanding and that can lead to one or the other getting in behind, and from that they set up lots of crosses and cut backs. If the winger hasn't got that support, he's more restricted.
I don't know where Styles best position is, but I don't think its full back. I think he's more of a Molumby type player, so maybe in midfield.