Just for fun here are all the #4 and #15s from 2000 to 2020, 21 and 22 are too soon to tell really...
2000 - Marcus Fizer vs Jason Collier
2001 - Eddy Curry vs Steven Hunter
2002 - Drew Gooden vs Bostjan Nachbar
2003 - Chris Bosch vs Reece Gaines
2004 - Shaun Livingston vs Al Jefferson
2005 - Chris Paul vs Antoine Wright
2006 - Tyrus Thomas vs Cedric Simmons
2007 - Mike Conley vs Rodney Stuckey
2008 - Russell Westbrook vs Robin Lopez
2009 - Tyreke Evans vs Austin Daye
2010 - Wesley Johnson vs Larry Sanders
2011 - Tristan Thompson vs Kawhi Leonard
2012 - Dion Waiters vs Maurice Harkless
2013 - Cody Zeller vs Giannis Antetokoumpo
2014 - Aaron Gordon vs Adreian Payne
2015 - Kristapz Porzingas vs Kelly Oubre
2016 - Dragan Bender vs Juancho Hernangomez
2017 - Josh Jackson vs Justin Jackson
2018 - Jaren Jackson vs Troy Brown
2019 - D'Andre Hunter vs Sekou Doumbouya
2020 - Patrick Williams vs Cole Anthony
By my reckoning, 4 absolutely destroyed 15 in 5 or maybe 6 of these years. 15 destroyed 4 in 3, maybe 4 years on a stretch. The rest were roughly even, imo.
So out of 21 years, 4 was the better choice 6/21 or about 30% of the time. 15 was the better choice 4/21 or about 20% of the time and the other half of the time it was fairly a crap shoot with busts and ok players coming in both positions.
Interesting. Of course a lot of this depends on your analysis of player impact and for me, just now, I did it on my gut, no stats. So take that for what it's worth.