Tough matchup. Duran is much smaller than Spence and that would make it tough for him to compete with such a physically strong and rugged welterweight. But, I do think that Duran would get nasty and get into Spence's head with words and "pro tactics" build from his insane number of fights. Too much experience for Spence to overcome with his physicality. UD for Duran.
I don't favour anyone over Duran at welter besides Hearns and the 2 Sugars. Maybe Mayweather has a chance because if his defensive skills. But Spence's aggression will play into Duran's hand.