The final 3 missions of the game are different depending on which ending you took. Both endings will reward you with $250,000, and either Kate McReary or Roman Bellic will be killed, depending on the player's choice.
The first three, if spared, all become random characters (in Clarence's he appears with a gun and attempts to kill Niko and Niko will have to kill him, it should also be noted that if Niko chooses not to kill him and drive away, Clarence will chase Niko. This contrasts to the other two which will request Niko's help with something). Killing Playboy or Dwayne both offer different rewards, killing Derrick gives you money but killing Francis does not give you money. The choice of Darko has little to no implication, while the final choice is the biggest of all - it triggers different missions and determines the fate of either Roman Bellic or Kate McReary.