Arenado is a much different case compared to Harper. I didn't have to talk to any scouts to figure that one out. For starters, Arenado turns 21 in April, which makes him a year and a half older than Harper and scouts still believe Harper will beat Arenado to the big leagues. He beat him to Double-A, where Arenado has never played and Harper hit .256/.329/.395 in 37 games with Harrisburg.
Of course, who sees a pitch in the majors first is essentially meaningless. Both of these players will make the major leagues this year (likely shortly after the calender hits June), and what matters at that point is how they do once they get there. Scouts also see Harper besting Arenado here and I'd be hard pressed to disagree with them, especially if Harper is a center fielder when he comes to the majors. Playing an up the middle position like center will give Harper even more value as a player who has the potential to lead the league in home runs. If he even approaches his power potential in his early twenties while he has the speed to play in center, he's going to be on the short list for best players in baseball.
Though scouts think very highly of Arenado, who has improved his defense at third to the point where he's an asset in the field in addition to being a well above average offensive contributor at third base, they just like Harper better. "Much better hitter," a National League scout said of Harper. "The other guy, though, does have a great swing for the Colorado park."
Still, the only scenario in which I see Arenado beating Harper to the majors is if the Rockies get off to some kind of really good start and he's crushing it down in Tulsa. If Casey Blake is out of the picture, that only increases Arenado's chances of being up before June, which would still be fairly slim. Rockies General Manager Dan O'Dowd would have to be desperate to improve the club and I doubt he'll show that kind of urgency that early in the season. Harper doesn't have this going in his favor as it's likely the Nationals will have a better chance to contend and Harper will have more to contribute next year. Even if Arenado makes it to the major leagues before him this year, Harper will arrive to the big leagues as a 19-year-old at some point in June and have beaten Arenado to the show by almost two years.
As pretty much every scout has said one time or another, "Harper will be first."