fangraphs has his babip there at .288. not gonna recalculate jes's to see who is wrong there, but i tend to trust fangraphs. I think an easy thing to miss when looking at babip, is comparing a prospects babip to what is expected in the majors rather than minors. In the minors, good hitters are usually more .340 to .350(admittedly, i'm pulling numbers out of thin air there, either way it's higher than the .300 at the major league level). If happ had matched his .347 he had at first stop, for example, he would have had a little less than a .300 average in the midwest league.
The question is, which hitter does one think he is? Of course, if you don't think he's a major league caliber hitter, then believe in the .288 figure. I think he will be a major league caliber hitter so I believe in the first number. He seems to have a good eye and solid power. A little swing and miss seems to be his main negative, but hopefully it won't be too harmful. I also trust the cubs current organization and scouting department. That gives me a little more faith in the first number.