The main difference between Garnett and Mchale is that Mchale was a 1-on-1 stud, whereas KG's impact is team-level.
Offense: Mchale was a better scorer with better 1-on-1 moves. KG was still an excellent scorer with excellent 1-on-1 moves, but still, say advantage Mchale. However, scoring moves <> offense.
Garnett, with his passing, had a global excellent impact on offense. He was a "point power forward" for much of his time in Minnesota, and his passing ability played a huge part in it. The Timberwolves built four consecutive top-6 offenses with Garnett in this role, maintaining an elite offense through 6 different starting point guards and multiple combinations of teammates (with no group having an abundance of talent), with Garnett as the only constant. Garnett was creating offense for both himself and his teammates, something Mchale never showed himself capable of doing.
Defense: Mchale was a versatile 1-on-1 defender, capable of guarding wings and post players effectively. Garnett was even more versatile, capable of locking up players as diverse as Tracy McGrady and Yao Ming. However, again, where KG distances himself from the man that drafted him is in his ability to make a global impact on the defense. Garnett's pick-and-roll defense, his ability to zone cover large areas of the court and then recover for the rebound, and his ability to play defensive quarterback are on another level from Mchale (or, really, anyone else).
Team role: As some have pointed out, Mchale proved himself a great lieutenant to Bird. However, he never had to lead. And while maybe he could have...we'll never know. Garnett, on the other hand, carried an expansion team to national prominence by himself as "the black wolf" and then came to Boston, where he immediately became the dominant personality among a group of otherwise strong personalities that have been perennial contenders for half a decade.
Garnett IS the Larry Bird of this team, and historically is closer to Larry than Mchale as far as level as well.