With McConkey, you're getting a guy who's ready and able to contribute now, instantly carving out a role but the ceiling is low due to physical limitations (at least in comparison to Leggette who's just as fast and explosive from a testing standpoint despite being 35-40lbs heavier).
Some people throw Garrett Wilson's name in there as a comp for McConkey based purely on measurables but he is nowhere near as dynamic as Wilson with the ball in his hands nor as acrobatic when the ball is in the air. To me, McConkey is a faster version of Hunter Renfrow which isn't a bad thing to be honest.
With Leggette, you're adding a kid who definitely needs some work on the subtleties of the position but offers you a rare profile to work with and near limitless upside. It might take him a bit longer to get up to speed but when it all comes together for him, I believe you'd have a guy capable of absolutely taking over games.
So, in the end, I think it boils down to the team, their scheme, the urgency of their need at wr, and how patient they feel they can be with the kid's development in year one. If you need to plug a guy in NOW then McConkey's your choice. But if you feel you have a WR corps in place that allows you the time to cultivate a rookie's potential then you're taking Leggette.
In my opinion, Leggette simply offers a skillset/physical profile combination that the Chiefs don't currently have in their WR room. Not to mention Leggette's film pops a lot more than McConkey's in my opinion. Those two factors give Leggette the edge in my eyes.
Just my $0.02.
