If Q is indeed omniscient, then he was already aware that Picard was capable of passing his test in All Good Things. Since that indicates to the Q that humanity are potentially on an evolutionary track that will result in them becoming equals to the Continuum, Q may have had a vested interest in giving them a slight helping hand - after all, the Borg aren't likely to evolve much further than they already have, humanity very well might.
I think that Q indulges in partial truths where his own omniscience is concerned.
Although he is able to predict and manipulate humans, he doesn't really know what they're going to do. I think his fascination with Picard has as much to do with Picard's overall maturity - there are buttons Q can push, but overall Picard is the most resistant to him.
Other advanced beings have indicated humanity's potentials, but in Q's case there is a real perplexment. Q draw their energy from chaos and impulse and almost just a pure evolvement of the id.
I think his motives in dealing with Picard and Co. always were to study that completely alien balance that evolved humans occasionally attain. In a way Q's mission was the same as the Enterprise, he is almost an inversion of the whole concept. One man instead of thousands, self interest being the rule, interfering and involving oneself being the cheif way of attaining experiences and information.
So I went with the Guinan choice. Though it may have worked out well for humanity. The Borgs' constant assimilation could have easily given them a total battle dominance which they nearly had anyway, had they encountered humanity in a few hundred years, it would be a roll of the dice whether humanity or the Borg would have advanced relatively further.