My opinion is Magnum T.A. Both were extremely popular and both were on their way to the NWA World Title when tragedy struck them, but Magnum had the more widely-available TV exposure, being on Ted Turner's TBS superstation. While David was pushed as an "all-American boy" (like his brothers), Magnum had the movie star good looks.
David's base of operations was WCCW in Dallas Texas, a hot but small promotion. Magnum's base was Georgia Championship Wrestling in Atlanta, a hot but much larger promotion. And with Ric Flair and Dusty Rhodes giving their stamps of approval to Magnum, and big stars like the Midnight and Rock & Roll Expresses, and the Road Warriors in GCW, the promotion itself got more attention and Magnum would have been the focal point of the entire promotion, supplanting Dusty Rhodes as the main face in the NWA.
I really don't think David would have had a really lengthy World Title run, because although he toured Japan, he didn't travel much outside of Texas, whereas Magnum traveled all over the southeast and the east coast with Crockett promotions.
Magnum no doubt would have had multiple World Title runs, feuding with Flair, Nikita Koloff, Barry Windham, Tully Blanchard, and other big stars in Crockett's promotions plus the other territories he would have traveled to defending his belt. Magnum would have been one of the main players in the NWA in the late 80's and 90's, possibly attaining the level occupied by Flair and Rhodes, and Harley Race before them.