CHARACTERS HISTORY (taken from Wikipedia):

Johanna Mason:
Sarcastic and mean-spirited, Johanna's first meeting with Katniss involves her stripping out of her tree costume in order to make Katniss uncomfortable. She reveals that she has no friends or family in District 7 during the 75th Hunger Games, stating "there is no one left I love." She won her original Hunger Games by pretending she was a weakling and a coward, so no one regarded her as a threat, but when only a few tributes remained, she revealed herself as a vicious killer. Johanna teams up with Katniss, Peeta, and Finnick in the Quarter Quell, bringing Beetee and Wiress with her. Katniss describes her presence as being akin to having an older sister who hates you. Johanna takes part in the committee to keep Katniss and Peeta alive, and helps by protecting Katniss throughout the Games. Near the end of Catching Fire she removes the tracker in Katniss' arm by cutting it out with a knife. Johanna is captured by the Capitol at the end of Catching Fire. In the third book, she is rescued along with Peeta and Annie and brought to District 13, where she fights with rebel forces and improves her friendship with Katniss. She is to fight in the Capitol with Katniss (although not as a sharpshooter — being from an area where lumber is the main export, her main weapon is the axe) but during training in District 13 she panics when the training arena is deliberately flooded. Katniss later learns Johanna has a fear of water, which developed as a result of her Capitol torture: being soaked in water and given electric shocks. Because she and Katniss refuse to be held in the District 13 hospital to treat their supposed mental illnesses, they convince the officials of District 13 to let them room together. Johanna is part of the committee that votes whether or not to hold one last Hunger Games using the Capitol's children: she votes yes.

Primrose Everdeen:
Primrose Everdeen, nicknamed "Prim", is Katniss's younger sister. She was 12 years old in The Hunger Games and 13 in Catching Fire and Mockingjay. Like her mother, she has blonde hair and blue eyes.
At the Reaping for the 74th Hunger Games, her name is pulled by Effie Trinket. Her older sister Katniss volunteers to take her place. As she leaves for the Capitol, Prim receives a promise from Katniss that she will do all that she can to win the Hunger Games. This promise guides many of Katniss's actions in the game.
In The Hunger Games, Prim is thoughtful, sweet, and somewhat delicate in spite of all she has been put through at a young age. She is a skilled healer, having been taught by her mother. In Mockingjay, Prim is singled out by District 13 to be trained as a doctor. During Catching Fire and Mockingjay, Katniss sees a change in her personality, as she has become more solemn and mature. Katniss once said that Prim is "the only person I'm certain I love."
In Mockingjay, Prim is sent into the battle for The Capitol to serve as a Casualty Aide by President Coin. During this battle, Prim is killed in a bombing, causing Katniss to go into a deep depression. President Snow later reveals to Katniss that the bombing was caused by the rebels to galvanize public opinion against the Capitol by making people think that the Capitol caused a bombing, then timed things so that a second series of explosions would detonate when the medical corps arrived to help survivors. Katniss already knew from Gale that this was a strategy being contemplated by the rebels. This information prompts Katniss to execute President Coin instead of Snow.

Yeah, I really like them both!

Johanna is my favourite <3

I love Prim, but Johanna is so badass! <3

I honestly never really liked Prim... yes, I know without her there would have been no books. I just could care less about her character.

Johanna is awesome

Both, but Johanna is amazing.