This is a good question, I had to take a couple minutes to think about it. I went with Trout, I think he will wind up the more complete player although both appear to be on their way to superstardom.
I'm biased because I knew Harper when he was still in high school but I don't think you could go wrong with either.
I voted for Trout. In my opinion he's a more complete player. Above average glove and range in the outfield, also had 30 HR's and 49 stolen bases last year. But Harper will be a great ball player for years to come, no doubt.
Trout is clearly better at this point. What the future holds, I can't say, but it's no contest if you look at them right now. Trout is better both offensively and defensively.