I'm realizing now that my question may have been a bit unbalanced. I forgot myself to consider the eras in which they played and the different focuses on the Center position.
My personal preference would also have been to go with Robinson, but I'm thinking that's skewed a bit by that being the era when I actually watched the NBA. (I stick mainly to college basketball, now.) During Robinson's era where the center role was just that, I will stand by Robinson as the better player; however, the game has now evolved a bit where the post-up center position barely exists. Center has evolved to more of an "extra taller power forward", and that plays better into Garnett's skill set. I still think Robinson would be a Top-5 center now, but I think Garnett would be considered better in this era (and vice versa).