If the Holmes brothers were to become criminal masterminds, Sherlock would be the superior one.

I will gladly accept this challenge.Your premise is both brothers turn to crime at some point in their respective careers, because you state the become criminal masterminds. So we are not discussing a parallel universe wherein both were raised to commit crimes, rather they started into their careers and then changed their mind.Based on this premise, Sherlock Holmes would indeed be the better criminal.The only one reason I will present is obvious. Mycroft Holmes doesn't understand crime the way his brother Sherlock does. That's why in the original source material, he consults Sherlock, instead of solving the crimes himself in a far shorter time. This happens in any and all incarnations of Sherlock Holmes. It's Sherlock who advises Mycroft and cracks the problems Mycroft has no understanding for.This means that despite his higher intelligence, Mycroft lacks the basic understanding of the criminal mind. He would hence make a great planner for another criminal, but a mastermind he would never be.A criminal mastermind is full of criminal ambition and genius. As you stated yourself, this is what Mycroft is lacking on all counts: ambition.Sherlock Holmes' intention is to always prove himself the cleverest person. If we take his strong morals away and have him turn to crime, he will stop at nothing to make his sinister schemes work, based on your own description.A crime planned by Mycroft would be a theoretical masterpiece, but in the end he would stumble over practical issues. Surely, he would be ready to sacrifice any of his men in cold blood, and eliminate anyone who might betray him, as he's a master at eliminating possible risks, as you describe. But this would leave him without a crew to execute his plans. Mycroft does not understand the criminal mind. Sherlock Holmes plays with the emotions and greed of simple folk, whenever he goes undercover.Mycroft's missions prior to his turn to the dark side were, as you state, directed against military, espionage organizations and such. This means he never had to deal with people desperate for food or money. He assumes people to act logically, and that would be his downfall. In the military, if you catch a thief, he will be court-martialed. This would be Mycroft's way of dealing with people inside his own organization who steal from him, too.Sherlock Holmes has repeatedly shown in the short stories that he understands that greedy people can be easily bribed, and thus be very reliable, if they are rewarded rather than punished. This way, Holmes in the stories has gained some notoriety and respect among simple folk, like his Baker Street irregulars.Mycroft would choose accomplices for his crimes based on their capabilities. Sherlock picked his troop of street orphans out of a different approach: inconspicuousness. He doesn't need particularly skilled people. He needs a few of them, but mostly he knows how to use simple persons to greater ends.A criminal mastermind must also know how to work with notoriety.Mycroft would keep his organization completely secret. But that would be a fatal mistake. There will always be people who have seen something, without knowing what it means. If later asked by the police (or their consultant detective), they will always tell what they saw, and someday, someone ill get the larger picture. That is how Sherlock Holmes managed to bring down Moriarty's network in "The Final Problem": picking up information and putting them together in his brilliant mind.But nobody ever rats out Sherlock Holmes, because of his reputation. Holmes would know that he had to hold the entire city in a grip of fear, having his street orphans spread the word that anyone who tells the police about Sherlock and his organization won't live to see the sun set. He would make sure that all persons who could spoil his plans would be fully aware of the consequences.Sherlock Holmes' return in "The Adventure of the Empty House" shows that he understand that concept, as he plays hide and seek with Moriarty's henchman Moran for three years, knowing full well that the man is out to kill him. Now if a Sherlock Holmes is afraid of one man wit a rifle, this threat must mean something. But Holmes was never openly threatened by the man. He knew Moran was out for revenge and would stop at nothing. He would let London's simple population know that speaking his name alone could result in sudden death, poisoned or stabbed by an orphan in the street.A secret service operative like Mycroft would never do this and would eventually be caught, because someone saw something he deemed unimportant at the time. Mycroft cannot eliminate every simple man on the street - he would end up killing most of London, thus attracting unwanted attention.So, this is my case. I'm ready for your rebuttal.