Last year, while playing for the Nationals' Single-A affiliate, the Hagerstown Suns, Harper went deep off of Zachary Neal, a pitcher for the Greensboro Grasshoppers, the Single-A affiliate of the Miami Marlins.
As you can see in the video, Harper admires his blast for a few seconds, then begins to trot around the bases. Neal, who took offense to Harper's admiration of his home run, is yelling at Harper from the mound. As he gets to home plate, the slugger looks at Neal and blows him a kiss.
Now yes, this incident was grossly overblown—Harper was a teenager who got caught up in the moment—but that doesn't change the fact that it happened.
To his credit, Harper seems to have matured. But we've never seen anything even close to this from Mike Trout, who is only a year older than Harper.
But there's always the chance that Harper could have a momentary lapse of sanity and show up an opposing pitcher in the show, and he's likely to receive far more than some choice words from the mound if that happens against the wrong guy.
So the edge in attitude goes to Trout, who as I said, we've never heard or seen act as anything but respectful towards anyone associated with the game.