depends on what you mean by "better atheltes". I think my way of defining is that if you put them across different sports, which group would perform better, and to me, that's NBA players. They have more more coordinations, and more stamina, probably more flexibility and balance too. Strength/explosiveness aren't the only parts about being athletes. Like cross country, water polo, archery, and etc, it's really hard for me to envision most of the NFL players do those better than NBA players. they would still be exceptional though.
Also, I could be approaching this the wrong way, but there are way more basketball players in the world (probably thousand times more), and in order to make it to NBA, besides skills, they have to be also top like 0.001% of the athletes, and compare that to NFL, which you might only have to be the top 0.1% of the athlete in the sport.
My another thought is that, and of course it could just be my bias, is that if all things being equal, you would probably prefer to be a NBA player than a NFL player since there is less injury, more money, and more fame. So basically most of NFL players, besides their love for the sports, probably did play basketball but was never the top prospects, which implies that despite being stronger and faster than some others who were better than them, they were lacking in some departs to truly excel the game. Again, I love basketball so this is an extremely bias take, I had done no research, and I am willing to be wrong, it's just a thought.