Stamina Points are the units of energy that all player characters have, which co-exists with their Health. They are usually referred to as just Energy, Stamina, or SP.
Stamina basically determines how long a person can run. Though it can help, it also has its disadvantages.
Stamina is affected by the gear and weapon that the person is currently using. When they Sprint, they use up their SP until they stop sprinting or they run out of SP. When that happens, the SP slowly recharges until it is full once again. While there are ways to increase the amount of SP one has (like the Black Beret), there is no possible way of speeding up the recharge process. However, certain items such as the Stamina Kit instantly replenish the SP bar.
Speed Vs. Sprint Stamina
In theory, a person with higher Sprint Stamina should be superior to someone with no stamina boost but higher speed. While it is true that people with higher stamina may eventually outrun a person with less stamina/higher speed, stamina doesn't affect how quickly one moves. In essence, the opposite may easily apply, as speed will actually get one where they want to go much quicker, and stamina only allows one to move there at a certain pace for a longer period of time. Because of this, players often prefer Speed over Stamina in any given situation.
Another drawback includes the regeneration rate of Stamina. While it is also correct that having a lot of sprinting energy allows one to run faster, once the supply of energy is depleted, one is left with nothing but their original speed and a long recharge time, which in turn can leave them devastatingly vulnerable. However, speeding up the regeneration rate may technically be superior to the combination of both Speed AND Sprint Stamina, because one can easily keep running and never have to worry about losing energy, which gives them the momentum to never stop moving.
Of course, a good alternative instead of picking just Speed or Stamina is always balance. Many characters combine high speed and a large amount of stamina (Like Viper, Raven, or the Infected), allowing them to run at a quick pace AND still be able to maintain that speed for a long period of time. Players often combine speed and stamina items together (such as the Light/Recon Vest and Shapka combination, or the Recon Vest/Tactical Backpack/Cyclops Bandana AND a Patriot Mask), giving them an excellent speed boost along with a decent amount of stamina.