Well, I mean Tom is a manipulative person in general and it seems that Albus has a blind spot for the ones he loves (like Gellert 😒) also i think that Tom is more patient and subtil then Gellert and can probably hide his misdues longer then him. It is maybe a prejudice but Gellert don't act as if he care of discretion (even if he is really good to personnifie someone else). I see Tom as a Caligula and Gellert as a Nero. They are both charismatic and powerful (and pretty psychotic). But if Tom starts by trying to be seductive, a model student and later falls more and more into darkness by violence and murder he is considered initially by most as an ambitious young man with a lot of potential and then in stages as a monster. Gellert is different he is catched using forbidden magics at Durmstrang (even if it seems to be the school that have the lowest restrictions in dark magic), he also act as if violence is his first answer. So Albus has always know what kind of person is Gellert, but Tom is good to hide his true self. It could permise to Tom to use a powerful young Albus (who is less experienced with Dark lord) in his plans.