If you want a consistently significant performer on both sides of the ball, you want Manny Machado. If you want a player with an enormously high ceiling along with below average defense and occasionally mediocre seasons, you want Bryce Harper.
In my recent infatuation with WAR, I was amazed to learn that Bryce Harper had a WAR between 1.1 and 1.5 in 3 of his last 5 seasons (literally every other year). For a player who is expected to earn the richest contract ever, that is quite a revelation. Of course, he did post a WAR of 10.