I think Colt McCoy is the choice, though I would not dismiss Harrell so easily. Frankly, I've never understood what the "system" tag means. If Texas Tech has such a great "system" that it allows people to put up crazy offense, why haven't more teams adopted it and done so? And just because it's a "system," that doesn't mean the other defense just lies down. I dunno, I think Leach knows exactly what he wants out of an offense and is fantastic at finding QB's who fit the mold he's looking for. As a matter of fact, all the recent QB's for Tech even look like each other. I wouldn't be able to tell who's who in a lineup.
But McCoy is the more complete player, IMO. He can move the chains with his feet, has shown leadership qualities, and is a great passer. I also disagree with the suggestion that Colt will not make a good pro. I think that many Heisman winners bust in the NFL because they are generally freaks with a truck load of raw talent. But that alone will not get the job done in the NFL; you also need a work ethic to work hard day in and day out adapting your game.
Colt has that work ethic (so does Tebow, for that matter). He does not appear arrogant and is willing to make the changes to his game that he needs to win. He is mature. I can see him being another Steve Young.