Here's the thing. Spartans, namely Spartan II's are extremely proficient warriors. Of you take ghost reviving out of the equation, man for man a Spartan II is far above a guardian. That said, Spartan II's are kidnapped as children, underwent mental and physical training that would be considered brutal to adult, and indoctrinated to be the perfect soldier imaginable, robbing them of a large part of their humanity. By the time augmentation is done Spartans are less soldier and more just a thinking weapon. I'll take space magic and retention of my humanity over being s better soldier any day.