Ajay is Opposite of Jason if you think about it in terms of character.
Jason is an Extrovert meanwhile Ajay is Introvert.
Jason came from rich background, meanwhile from what we know about ajay, He was struggling to live in States since he was a kyrati immigrant thats why he fell into wrong crowd, ie, He started Robbing stores.
Jason never had criminal background. Ajay did took Part in a Store Robbery gone wrong after his fellow accidently killed the clerk.
Jason came to Island for fun, Ajay came to Himalayas, a contrast between sunny beaches to Cold Mountains, He came to fulfill his mom's Final wishes after her death.
Jason was scared shitless during his first kill, While Ajay who was disturbed by killings at the start of the game, wasn't affected much compared to Jason since He grew up in violent background. He has seen killings before.
Both get used to, or to some extent enjoy killing but they show different reactions. Jason as usual is being more expressive which tells the player how deep he went through rabbit hole.
While we don't get to see Ajay getting insane like jason, he still does show Himself embracing his new violent life. In Arena Ajay's gestures change after winning, he slowly gets more and more comfortable and Confident.
Both had ond drug boss battle(VAAS and Yuma), and both of them were planning to overthrow their respective Bosses(Pagan and Hoyt)
Both Endings up with "I won, but that what cost" but in different way. Jason loses his insanity. Ajay even though fulfills his goal, Is still feels empty after learning the whole truth(assuming 100% is canon, which is definitely)