Reinhart's been a better goal scorer but it's worth noting that a lot of that difference is coming on the powerplay, where Reinhart gets 2-3 times as much time as Coyle does. At even-strength things look better for Coyle:Reinhart has hit the 20G mark in each of his 3 seasons. Coyle hit that mark once, when he was a year older than Reinhart is now. Reinhart is more of a playmaker than sniper, but he's still better than Coyle at putting pucks in the net.
Coyle
2015-16: 82 GP - 21 G - 19 EVG - 0.23 EVG/GP
2016-17: 82 GP - 18 G - 14 EVG - 0.17 EVG/GP
2017-18: 66 GP - 11 G - 10 EVG - 0.15 EVG/GP
Reinhart
2015-16: 79 GP - 23 G - 15 EVG - .19 EVG/GP
2016-17: 79 GP - 17 G - 8 EVG - .10 EVG/GP
2017-18: 82 GP - 25 G - 15 EVG - .16 EVG/GP
And again, 2017-18 was the season that Coyle had injuries in both wrists that affected his shooting pretty badly, to whatever extent one considers that a factor.
I haven't looked at the quality of the team's powerplays or anything like that. Minnesota's has infuriated Wild fans at times, but my understanding is that it's been average to above-average for the last 2-3 seasons. Coyle faces more competition for powerplay time, which probably explains the difference in PPTOI between the two. I don't necessarily think Coyle's goals would jump past Reinhart's if their PP time was even, but I think it'd be pretty close.
There's no getting around the fact that Reinhart is 4 years younger than Coyle and still has quite a bit of room to grow, especially if the Sabres can improve overall. Coyle's faced harder competition for top-6 and powerplay time, but he's also played on better teams.
Edit: To be explicit - none of the above is meant as an argument for this trade making sense for the Sabres. The age difference makes it pretty clear that they don't do it.